Apple does not want to support two sets of tools forever, it would not be cost effective.
GCC is obsolete on Apple's platforms because Apple has chosen LLVM as the future basis of their tools. The reasoning was mostly technical, LLVM project also seems more nimble and aligned better with Apple's goals. Surely saner license did not hurt.
LLVM has technical benefits compared to GCC, biggest being modularity, speed and memory usage come second. Modularity helps Apple make Xcode a better IDE. You can already see stuff in Xcode that would have been very hard to make with less modular toolset, as GCC. I expect it to pick up after the transition period is over.
As an example they are using clang to parse the code for syntax highliting and code completion instead of a custom parser they used before. To use one codebase for parsing both in the IDE and compiler has inherent virtues.
LLVM is also used by Apple in other products in addition to of Xcode. OpenGL runtime and OpenCL tools come to mind, efforts in LLVM project have good return on investment.
Everyone, old or new projects should consider switching to clang, for speed and static analysis. Whether to use new objc features that require clang, is another question, in most cases it makes sense.
