It is in Apple's best interest to have a compiler that is also used to do the checking of code in real time within the IDE be the same. It makes more sense because otherwise you get differing results in the IDE versus compile time. It is within Apple's best interest to help developers develop and make it easy to find issues, clang's output for errors are absolutely fantastic. If you type something wrong it provides suggestions for what you may have meant (and most of the time they are accurate).
I think Xcode is one location where Apple is showing that it does care about developers by building a better product with better integrated tools (git for example ...). Are there things we miss from the old Xcode, sure, but overall it is clear that Apple understand they need to provide developers with tools that help them get their job done.
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Clang though is helping the ENTIRE development community. It is an BSD licensed compiler with some absolutely awesome features.
1. Their human readable error messages. I've spent a lot less time Googling random compiler failures or trying to parse GCC's or MSVC's output so that I can understand it and fix the error (especially in heavily templated C++ code) just by having the compiler give me human readable errors. This goes a long way for a developer that misplaced a semicolon or is attempting to use a const variable into some Boost code that expects a non-const variable thereby causing thousands of lines of template errors to be thrown. 2. The LLVM allows for very interesting "languages" to be easily prototyped and tested, and JIT compilation and a few other things can be created. 3. clang can read incomplete code files and make code completion suggestions. Not just tied to Xcode, but as a library. clang_complete for Vim is absolutely fantastic and by far my favourite Vim script.
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Why exactly should Apple be required to ship GPL versions of the software when they could ship BSD licensed utilities without having to worry about being GPL compliant. Just because a few people prefer the GPL version they can't remove it from their system? They believe the GPL licensed tools to be inferior (and with the GPLv3 causes more legal headaches than it is worth) and thus they are replacing them with a set of tools that fit their requirements. Why does that mean that they "could give a rat's ass about the developer community"? Why exactly would they continue to develop clang/llvm as an open source project?