There is when they have significantly different properties/workflows. I happen to think Darcs is supremely good for web development (probably not at facebook scale though)--it's the triviality with which you can cherry-pick patches that makes it wonderful to fast track bug fixes up to the live site while sitting on bigger, more untested changes.

I use Git when I'm releasing binaries to people (because I like its tagging better) or when I have to track some upstream code and merge in some local changes.