So it doesn't completely displace most compiler, but it does displace a lot? On the flip side, how many people actually use Intel's C compiler as opposed to people who just use GCC?
On Windows, GCC is not a very viable option (yes, I'm aware of Cygwin, MinGW, etc) for production software right now. And you'll be hard pressed to find a solid compiler that ISN'T proprietary.
On Windows, perhaps due to competition from GCC, perhaps not, Microsoft offers various Visual Studio Express editions free of charge, and the license allows commercial use, so you can sell programs made with Express editions. You're right in that the compiler IS proprietary, but the question is not of license, but of wallet cost to user - $0.
