I'm surprised that nobody thinks this is a good idea. To the poster above that said it's like a car with helicopter blades, I have a better analogy: This is like a car with two motors. One motor is street legal and can be driven in all fifty states. The second is a fully modified fire-breathing 800HP monster that can only be used in closed-course racing. When you're driving to work you use the street legal motor, but you can drive the same car to the race track and get the full potential of the second motor.
As MP3 players get more and more storage space, we're going to see scenarios similar to those in desktop computers - Grandma only needs a fraction of that 500GB drive in her new eMachine. The same will be said for Sally the high school student with her 60GB iPod. If that space is available why not fill it with the highest quality music possible so that music is available wherever one goes? I understand that it won't be playable in my iPod but it will be available to hook up to a stereo or computer etc etc at a friends house / party / barbecue.
Am I alone?
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.
When you don't know what to do, walk fast and look worried.