You're confusing products a bit. The two pieces of Myth that handle non-tv media, MythMusic and MythDVD. These are both plug-ins to Myth and while they do tend to ship with most Myth setups, they are not part of the core MythTV product. I agree with you that they both suck - The UIs and overall management of media are just terrible - But don't poo-poo MythTV because of them. In essence it's like saying Firefox sucks because your Yahoo toolbar has a bad interface.
XBMC is awesome but it's not a replacement for MythTV. It has no mechanism for recording tv shows or hooking to any type of tuner card. What it excels at is, as you said, media management. I modified my MythTV menu to include XBMC, so when I want to watch my DVD ISOs on a Windows box I launch it from the MythTV interface. A quick blurb on how I did it is here.
FWIW, you can add MythTV as a video source in XBMC using the mythtv:// prefix. You can then view recordings as well as live tv, although it was buggy the last time I tried it.