Pants as protection?

I have to rush to work soon, but I went to bed wondering about this question, so I'm going to ask all the parents of older boys...

When it is time to go back to wearing long pants, the kind that covers the knees, will those pants serve as some kind of protection against the skinning of the knees? Or should we plan on him ruining his pants with all the falling down and skinning of the knees?

Seriously. Do we need to get more 3T pants at the thrift store in preparation for fall ? (no pun intended)