MPlayer doesn't fail, you idiot. While libass might not support some obscure ASS tags and renders things a bit differently (which isn't really a fault, but since ASS subtitles are usually designed with VSFilter in mind, it might lead to some problems with libass), libass is a very good ASS renderer. And it's even faster than VSFilter.
MPlayer in general also has some strong points that CCCP doesn't have, like being standalone, cross-platform and portable. MPlayer in general is also more lightweight than CCCP is.
That said, if you're on Windows, you're better off using CCCP. Any other platform, MPlayer is the way to go. Now that there's also an ordered chapters patch for MPlayer, it probably won't take long that it arrives in the official version too.