I really appreciate your question. It got me to thinking, and sometimes that is a good thing.
First of all, I was thinking, what does a person mean by [when it is ready]? Does that mean ripe, fully broken down, or simply ready to use. Then the thinking continued about organic matter in soil, humus, and the equillibrium between humus loss and humus build up. I would assume that the addition of compost serves many purposes, but the build up of humus must be high on the list.
For me, compost is ready, when I'm ready to use it. What is composting after all? To me, it is the collecting of organic matter, placing it together, and managing it until the material breaks down until nutrients become more readily available. The main problem with adding compost before it [is ready] would IMO be the loss of nitrogen as breakdown continues in the soil. But isn't that what happens anyway, no matter what level of decomposition the compost is in when placed into the garden? The organic matter continues to break down until eventally it is no longer there, and what organic matter is there at that point came from the continuous inflow of orgain matter to the humus cycle.
While thinking about your question I did a little on line research and stumbled across what appears to be a great resource. I think that it is an e-book so won't post the link here. But the book is called Organic Gardener's Composting by Steve Solomon. The author has what may be some radical ideas, not sure, as didn't investigate, and they are not the focus of this particular book. But his chapter on maintaining soil humus and nutrients is very informative. He also gives an interesting recipe for adding organic fertilizer that is made from 4 parts any kind of seed meal, one part bone meal, one part lime, and 1/2 part kelp meal. Claims that it will do wonders for most soil when applied at a rate of 1 gallon per 100 sq. feet, plus a couple of handfulls under transplants or under hills. This non copyrighted book can also be viewed at a site called soilandheath.org or you can just google the title and author. It tells how to make really good compost, but goes further with how to enrich the soil using organic methods. I'm going back and read the chapter more slowly and will skim the whole book as it appears to be very informative.
Anyway after all of this rambling, here is what my compost looks like after being started last winter. I don't turn the pile much and so the piles do not generate a lot of heat and the breakdown is slow compared to a hot compost pile. Also, I am no kind of compost purist, but do keep a steady supply of organics going to the compost pile and then moving toward the various beds and plants.
Leaf fragments are constantly getting scattered among the ripe compost.
A bigger pile, and once again, the leaf fragments are a bit misleading. They are perhaps only 5-10% of the volume of this mostly very dark crumbly stuff that has texture about 3X coarser than premium potting soil.