I think I know the real reason why. The truth of the matter is that quite a few F2P users likely pay to become premium for one very simple reason...
Profit.
I've recently gotten into the Stram trading of the SPFU, and I can tell you that the random drops can get you a decent amount of scrap (quite literally). A single hat gets you at least 1 refined metal in trading, and that's only if it's sold as a craft hat. 1 Refined metal equals out to about $.85-90 if you want to sell it for cash. I once traded 10 refined for a copy of Saints Row 2. And if you get a really good hat, a misc item, or a new drop, you can get a lot more. Some items go for around 4+ refined.
And for genuine items (the kind you get from preordering games) can be sold for quite a bit. The pre-order items from Football Manager 2012 are going for about 1.5 buds right now, which is just about enough to pay for the game by itself. So those items you just have sitting in your TF2 inventory that you think are worthless? You might be sitting on enough to get you a new Steam game.
As for me, I mostly stick to game trading. I've made about $150 dollars worth of profit (not in cash, but in games I wanted to get anyways) already. I've gotten Skyrim, Serious Sam 3 BFE, Bastion, and many more. The only problem is it takes a long, long time.
And it's not just actual cash for tf2, but the fact that if you go ahead and pay fifty cents in the shop, you can get better items for the rest of the time you play the game. That by itself is enough to get many to just throw $1 or so into the game.