It's funny how the purpose of .maps isn't actually documented anywhere, and I just learned this stuff over time.
aaron, the RMF is a file format made to be worked with in an editor. The .map format was designed to be compiled, but that doesn't mean they're going to restrict you from viewing these files in the editor... it's just not ideal.
Also, considering how large RMFs can grow to be, it's possible that there were technical limitations at the time regarding how much memory was being used to compile. Thus, the .map format may have been easier for the computer to work with since it's significantly smaller. Remember, GoldSrc is based off the Quake engine, which started being used in 1996. Back then, RAM wasn't plentiful, to say the least.