There's a bit of a middle ground here
Using compiler extensions when they aren't needed really is a bad thing. Using them when they are needed, is not.
Clang already supports many of the GCC extensions because everyone recognizes that they are useful, and it's nice for the two projects to be compatible. They've also said that a few were poorly designed and will not be implemented.
As LLVM matures compatibility will continue to increase, and I haven't really seen any projects that are refusing to take contributions to fix up Clang compatibility. If something isn't working correctly, just patch the project with #ifdefs to get it working. That's one of the benefits of being open source, that anyone with an interest can go in and fix up their pet peeves.