Toenails seem pretty useless, and in fact more often than not just seem to be a source of trouble. Given that hominims have been bipedal for over 2 million years, it seems like toenails would have disappeared if they had no positive function at all. So, presuming that paleolithic humans would have derived some benefit from having toenails, what could that benefit be? Fingernails are clearly needed for additional gripping power. Toenails aren't strong enough to provide climbing grip (or are they?)
I use mine to scratch my calfs when I'm in sandals standing in the checkout line. Couldn't do without 'em. Now the question is, why didn't evolution fix our itchy calfs? Or our antsy compulsive behavior, which is probably more truthful.
Natural selection works slowly enough that our genotypes preserve traits that might come in handy with future physiotypes. Our descendants might someday fight like velociraptors, using their toenails to slash each other's guts out, in order to advance further in the checkout line. Sounds like fun, eh? I know I look forward to it.
Things don't disappear because they are no longer useful. Evolution is the process of selecting against things that have a negative impact on survival. Do toenails have a negative impact on survival? No. In fact, given the state of modern medicine and styles of living, very few things do.
Here's a discussion on this topic: http://www.thenakedscientists.com/HTML/content/latest-questions/question/2588/
I know that when I used to run quite a bit, my toenails took a beating. Some of the mega-runners don't have them. My point? I don't guess I have one.
Here's a picture of a toenail:
Change is ALWAYS happening. It just takes a lot of time for it to happen.