Because you'll spend a lot of time chasing down bugs that are actually issues in the compiler. Unless you're trying to vet the compiler itself, this won't improve your software but cost you a lot of effort.
Two things though:
1. Hopefully none of your users run an unreleased clang from head, so you're not using anything a user will run into.
2. Consider if it is it worth the effort to validate every commit pushed to head, or whether you'd get more out of your investment if you only tested releases that received some more vetting (e.g. nightlies).
Yes, but clang eventually gets released. You want to catch bugs in clang and get them fixed before that release happens.
It is not the same as playing with a release candidate.