When I was in school, everybody knew that stretching was a good idea. That is, either we had personally ripped something or gotten awful cramps because we didn't bother to stretch out before doing something vigorous, or we'd seen other people make that mistake and decided to be more careful ourselves. The connection was really sort of obvious.
Currently, I use stretching in small bits scattered throughout the day, because if I don't, my butt welds itself to the chair and my muscles lock up. Stretch, or ache. This also is really fucking obvious. Some people's bodies don't knot up as much as others, but if you sit long enough -- and people nowadays tend to sit a LOT -- then eventually it will happen. It can be fixed by stretching.
Anyone who's done a bendy sport -- cheerleading, gymnastics, bellydancing, yoga, etc. -- can tell the difference between people who do some kind of stretching regularly and people who do not. Granted, there is a strong genetic factor in how stretchy your body is, but it will certainly bend better if you stretch it regularly than if you do not. And if you haven't done it recently, you can sure feel the difference when you try to do it.
WTF even, science. >_<