Honest, my take is: You have to have humus. If you have no other source of humus use the leaves, use weeds, use what you have. If you have nothing, you will have dust, not soil, it will blow away!
In an ideal world, have a lawn to mow, have a compost maker, and simply pile manure hay, leaves, grass clippings, weeds, kitchen waste into it, and take wonderful compost out of it.
Most of us are not at either extreme, we have more than leaves, and less than the compost maker. So, we find the combination that works for us.
I have woods, I bring in a chainsaw and remove the large trees, then I bring in goats to eat the brush and get the bramble cleared out, then I run pigs to go through and root out the weeds the goats didn't get and get to the "root of the problem".
Then I go through and plant a grass mix, it will have alfalfa, fescue, rye, lespedeza, Kentucky blue grass and several other grass or legume type seeds. I let it grow, and self seed for one season, only one or the big weeds will return.
Now I mow and rake it, I let it set and dry out, and chemical changes happen within the grass. And finally I come back and bale it up. I have hay. But, within my bale of hay I also have weeds that grew in with my grass, small trees, and even some bramble and other stickers, and this is in a fresh field.
I spend some time grubbing out the stickers, and trees etc. But, next season I will still get a mix of grass, legumes, and brambles, trees and weeds. And some of the grass seed mix will take off and some won't. Some is cool season grasses and some is warm season grasses, so it depends on which cutting we are discussing to know what constitutes hay.
Straw: When farmers complete harvesting wheat, rice, barley etc. There is stubble left in the field, they then come in and cut it, comb it into windbreaks and wait for nature to do the chemical change, and then they come back and bale it. Yes, again, there will be some weeds mixed in, and maybe some grass, but not alot. Quite often straw is left long past the chemical change and it is more woody than is hay, and is therefore slower to break down.
I know how to make hay fields, straw fields and bales of either.
I also know that within the plants there is a chemical reaction, where the last of the chlorophyl is used up and the sugars turn into starches. It is not all simply decompostiion happening to the plant it is also happening within the plant.
To a lesser degree the same thing is happening to leaves, but they have already passed some of the stages that the hay or straw has yet to get to, and some of the biological processes, just aren't gonna happen there.
Even within a goat, the processes are happening. A goat does not digest their food, not really. They eat, they put the food into a holding stomach, where it is wetted and wilted. Then they bring it up, all full of the bacteria, and they chew it finer, then swallow it into a second stomach, where the microbes are heavy and very healthy.
Here is where digestion takes place. Here is the microbes that you must be very careful to keep in balance. A sudden diet change and those microbes are simply not there to digest the food, and the goat can starve with a full stomach.
Some of those microbes only digest grains, some hay types, some leaf and woody stuff. And they are inefficient at best on other types of greens.
If a goat is in trouble, you must give them probiotics, which are just some new bacteria to populate their stomach and digest the food. But, it is simply better to just make all changes more gradual and keep those microbes healthy in the first place.
Soil is similar, it has bacteria that digests and again, it must have sufficient numbers to do the job, or the stuff just sets there. The good news is, the soil isn't dying it is waiting for new little bacteria to get born to get the job done, more water, warmer temps and you got it, new bacteria. And straw or hay is similar to the probiotics that is used to rescue goats from malnutrition. You could give the goat a bit every day, to ensure they have plenty, but normally a healthy goat will make their own. I say that it is similar, what I mean is it is full of the bacteria, microbes etc and they are replenished, restored and fed by it.
You can make your own hay or straw at home. It does not have to go through a baler to become hay or straw. It does need to be cut, then cured, which is a process happening within the plant. The chlorophyll must be used up. And you have hay or straw.
To make your compost you are wanting: manure- teaming with the microbes to kick off digestions, hay or straw- plants that have had a chemical change, and greens- plants that still have sugars and have not been changed into hay or straw yet. A balance is what you want in a perfect world.
Again, we are mostly not dealing with a perfect world.
And I do use hay, and I use manure, filled with bacteria and microbes from the goats digestive system, and the bedding that is filled with manure, urine, and beginning to break down already. I can prove that I use hay, by the amount of grass growing in my beds! I always manage to miss a few seeds when waiting for the grass in the bedding to seed out and die.
But, I refuse to burn the leaves, and they have to be somewhere, so they are in the garden. And I only bring bags of leaves from my yard to set and break down in plastic bags, all the rest of the leaves used in my garden, put themselves there! I don't go looking for them. If I wanted more there is plenty and easy to get, but I don't ever need to go find any.
I have a 4'x4'x3' bin in the garden, it is suppose to be a compost bin, it is full of leaves every year. I also have a boat, that got hit by lightning and cant be repaired in the garden to hold compost, it is also a leaf bin. I am in the process of getting a shredder, then those leaves will get ground and they will break down faster. In the meantime, it is store them, use them, burn them, or bag them up and ship them to landfills. So, I store them in compost bins, until I can use them. And I use too many, and I still can't get ahead of the leaves that fall naturally into my garden.
Will I build a compost pile? NO WAY! And there are many on here who are not going organic, nor will they compost. But if they will just add some humus to their soil, whether it be leaves or hay, straw or grass trimmings, it is still better than creating a dust bowl. And mulching can be done in any situation and it is better than bare soil, whether the mulch is hay, straw, leaves, newspaper, or cardboard! I actually had a man on another forum, tell me to get rid of the leaves, and rocks and chop out those roots and stumps, and get that dirt some air! He was dead serious and raises a great garden.
I do not believe that it is a lack of intelligence, that leads one man to walk one path and another man to walk another. And I do not believe that the plants can only live if you do exactly like person A.
trying to make this soil into something that it isn't. I am going to try to make this soil the best that it can be, within what it is. I hope that makes sense.
-- Grass clippings ARE the hay sub for small-scale gardens like mine. So, WHY should I grow only LAWN GRASS? Why not grow more nutrient dense pasture or hay seed mix in a designated area and allow them to grow much longer (better for preventing matting down, etc.) for harvest and use? I already have the Orchard Grass area, which is along the ENE foundation of the house. The rich dark green of that "Kentucky Blue" make me think it's pretty good source of GREEN so it can just keep on growing there, and I could start a Clover Hay Mix in a sunny dry area. 
)
) I may have been making my veggie beds overly fungal with all the fall leaves and wood chips for mulch. I'm hopefully correcting that a bit with my "direct extract" almost compost tea. Maybe a little later I'll even make some real AACT (though I'm still finding the whole bucket, pump, time it for 36 hrs and use right away, sterilize all the equipment stuff not in real harmony with my lazy gardener, don't-work -too -hard- at -this style). But using more green mulches would be a good thing and not hard at all.
... I may end up with more hay than I'll know what to do with.... 
)
I'm still thinking of adding leeks between the toms and maybe a couple of bush beans.... 
Some worse than others. The big mystery volunteer tom in the back is the only one completely unaffected (It was the only one planted from a 1 gal pot. It's at the lowest point in this sloping bed.)
