We are not totally telling you different things. Everyone has to figure out what works for them. I do harden my tomatoes off, partly because I put them out very early. I am often setting my tomatoes in the ground well BEFORE the average last frost date (since weather often warms up before then, even if it might have a cool snap later). Putting them out early, they really need the hardening. One of the things that happens with spindly leggy tomatoes is that I lose a lot of them in the hardening off process. The merest wrong wind or handling them wrong while moving them in and out and those spindly stems snap right in half. Oh well, gone seedling... The early start is how I am eating ripe tomatoes in June in Ohio.
(I also transport my tomato plants back and forth in a car several times, because a number of them get sold at my church plant sale, meaning lots more opportunities for the spindly ones to get snapped.)
But 2c is right that it helps to give tomato plants more root system. My tomato plants are transplanted three times: once moving them from being very crowded on heat mat to one per cell off the mat, once potting up, and once planting. Occasionally if they are started early, there's another time, moving from 3" pot to bigger pot. Each time they are transplanted, they are buried deeper than they were before. So even though they are sturdy and stocky, they have been given some extra root system that way.
And I have grown sturdy ones and leggy ones. I can tell you about the snapping in half, because I have experience with them getting leggy too. One year when I planted seeds too early, I ran out of room under the lights and tried putting some of the tomato seedlings on a window sill. Wow! They got amazingly tall and leggy almost over night! So not only was that the year a lot of them got snapped in half. But for me, the really leggy spindly ones got off to a very slow start compared to the sturdy ones, even after getting planted. I attributed that to having a lot less leaf surface than the better developed ones, to collect energy with, but that was just a theory. Might also be more of a system shock, all that stem put into soil that is still pretty chilly. But they did survive and grow into plants that were eventually pretty indistinguishable from any others.
So (sorry, this is turning into a book) in conclusion... if you are putting your tomatoes out later in the season, not needing as much hardening off, and putting them into warmer soil, the legginess may at least not make so much difference or cause harm and perhaps would give advantage, especially if you were comparing it to plants that had not been through the transplant three times routine.
Hope this helps make it all clearer.... Thanks everyone, for making me thinking this through in more detail, so I understand it better!