The problem isn't that it's a big annoyance. The problems are:
a. it's error-prone. People who don't use MacOS every day will forget to include a case for "darwin" in a great many GCC-specific build descriptions. The two toolsets have a lot more in common than they have different, so most of the time, that will result in wrong builds.
b. It's a terrible name. "Darwin" is the name of the platform, and is not a good description of the toolset.
c. Most new BB users will not think of using a toolset named darwin when compiling with gcc on MacOS X. I've already seen several posts where you had to tell a user, "use the darwin toolset, not gcc."
When configuring gcc to target MacOS, the default assumption of the build system ought to be that it's Apple GCC. "Nobody" uses FSF gcc on MacOS.
I am completely in agreement. If you're going to have a build system, it should do the most reasonable thing on the platform in question by default. The vast majority of OSX users will have installed the free XCode IDE and associated developer tools provided by Apple. Boost should build correctly by simply issuing "bjam --v2" on this platform as elsewhere. Matthias