You seem to be implying having a competitor to GCC is a bad thing? While there is something to be said for duplicated effort, I think it's actually helped improve GCC a lot because they also 'copied' good things Clang did (like much better warnings, built-in static analysis etc.). So really everybody has benefited.
It's good to have a bit of diversity and options, especially when they're all trying to be compatible.
Having two good compliant compilers is great for C++ too: it keeps both compilers honest and also makes spotting weird behaviour easier in big codebases (i.e. to first order if your code results in different behaviour in GCC or Clang then it could be dangerous).
Also has the added benefit of reducing the need to use compilers like Intel's (not sure on current benchmarks) but I really wouldn't want to ship something for AMD CPUs with Intel's compiler.
Besides GCC, Clang, and MSVC++, most other C++ compilers license the EDG frontend, so there's less heterogeneity than you would expect. I don't actually know if there's another implementation of C++11 besides the ones I listed. Even Microsoft now uses the EDG frontend for Intellisense, rather than their own (more incorrect) frontend.
> It's good to have a bit of diversity and options
It can be consistent to hold that those two statements do not hold in this case. GCC requires that developers uphold certain user freedoms (i.e. compiler extensions must be free software). Clang allows user's freedoms to be more easily violated.
It's fine if you want to hold those two opinions, but if you state them as if they're the only opinions, that's not great. If you don't acknowledge that they're predicated on beliefs such as "user freedom isn't important to the expense of software improving in other ways" (or such), then of course you'll have trouble understanding why one might view clang/llvm in the negative light of effectively enabling "GCC, but without GPL".
It's good to have a bit of diversity and options, especially when they're all trying to be compatible.