gcc has no equivalent to clang's externally exposed AST access and navigation API. Therefore supporting clang as a semantic source does not constitute encouraging use of clang over gcc.
It saddens me that FSF doctrine would deny the emacs community a big CEDET step forward in functionality and performance. Especially when at least some competing IDEs are exploiting clang to provide features and performance that currently CEDET cannot match.
All projects on which I work (both professionally and in my free time) use the gnu toolchain. I have clang installed only to be able to use rtags. Does that make me an imperfect supporter of the FSF's goals and agenda?