Part of it is also that many graphics cards only support things in power of 2 sizes, and thus if you choose to have a 100x100 texture then it still takes up 128x128 in the card. Thus people fill it completely to get higher quality without penalty.

Also, I wish computer modular arithmetic worked like real modular arithmetic. Then you wouldn't need to say (x+size)%size, you could just say x%size which would be much cleaner to look at. But sadly, -1%3 equals -1, not 2, when doing this stuff on a computer. I ended up writing an alternative Math routine class to fill in this kind of hole...