Just to note... A few people mentioned that OSX already comes with a home theater "front end" built in. That's true, I suppose. It has a feature called Front Row now, that can, in a pinch, serve that function.
However, it hasn't been updated in years, and it certainly doesn't even come close to comparing to MC's Theater View. Also, since many of you aren't heavy Mac followers who try the betas and follow the threads, I thought I'd pass along: Apple discontinued Front Row in the forthcoming (next month) Mountain Lion release of OSX. It is being removed when you upgrade. And, if you know anything about OSX release uptake rates, just about everyone will update by the end of this year (especially since it is only $20). EDIT: I was wrong (I didn't realize because all my Macs are still on Snow Leopard). Front Row was discontinued and removed with Lion. It is already gone.
I also disagree that "competing with iTunes on a Mac would be too hard". The same could be said on Windows. Windows comes with Windows Media Player, after all, and iTunes is ubiquitous and free. Plenty of companies "compete" with Apple branded or built-in software by offering things that are in the same category, but do more.
Now, if MC was just a clone of iTunes, with no substantial additional value, then I'd agree. But it isn't. At all. It does WAY more than iTunes, and does it WAY better. That's like saying there's no reason for Adobe to make Photoshop and Premiere for Macs because they already come with iPhoto and iMovie. That's just absurd. Like I said before, most of the Mac users I know personally (who, generally, are "pro" or "scientific" users, not "grandmas") loathe iTunes. They love iPhoto and Keynote, but iTunes is not a favorite. And it isn't a home theater front-end, and it barely handles video (and not well).
Also, as a side note, I think one of the most appealing features for many Mac users would be Theater View. The aesthetic of Theater View is already VERY Applesque, and it would work splendidly with the Apple Remote that comes with most Macs (since it only requires the arrow keys, enter, and the "green button" that could be remapped to the remote's menu button).