It may be convenient in certain cases (such as the bots) to have a flag on build-webkit that filters its output through filter-build-webkit for more human-readable result. Though, keeping the scripts separate is useful so that old build logs can be filtered and so that build scripts other than "build-webkit" can use filter-build-webkit (in which case, it should probably be renamed).
One of these years we really should fix this. :) I don't see the mac port moving to ninja anytime soon, but xcodebuild is suuuuper verbose.