Aye, udeups put it better than I could: Keeping it as internal to MJ as possible is best.
I, too, don't want to worry about keeping a directory full of the files that I want to synch with (although that's kind of what I'm already doing... exporting my MJ playlists to a directory and having XPlay synch with it).
Ephpod and XPlay can already synch with files and folders on my hard drive. The one thing that they can't do, though, is interact with my MJ data. To me, the whole point of an MJ plugin is to do something with MJ that you can't do with anything else: make the iPod interact directly with my MJ data. More specifically, my MJ smartlists. If I wanted to set up synching with files on my hard drive, I'd just use Ephpod.
MMJB, iTunes, and XPlay are all geared to synching with their internal playlists (well, XPlay synchs with Windows Media Player's internal playlists) -- but there's one thing that's different: MJ doesn't suck.
The one thing that's missing from my iPod experience is having MJ do what the other iPod solutions do -- synch with my internal MJ playlists. MJ smartlists are truly innovative, and are the center of how I listen to music.
Gosh, I can grovel with the best of 'em, can't I? I apologize for my tone, Steve. I know that I'm sounding like a jerk, and you gotta know that I'm not trying to be preachy or anything. You know that I'm grateful for the hard work and long hours you're putting in on this thing. I sure as heck couldn't code anything like it in a million years, and I haven't the first clue on how easy or how hard it would be to do what I've suggested. I just want to very strongly voice my thoughts on this matter.
I hope that you've taken my suggestion on playlist/smartlist synchronization to heart -- I think it'd be a mistake not to do it.