I can definately see it that way... EG, the Market Share vs. Effort argument. However, I can also see it another way. For example...
I am a user of three different system types: OSX, Windows, and Linux (Fedora currently). Since you mentioned it, I myself use FireFox. All the security stuff and tabbed browsing and all of that is all nice, but the real reason I use FireFox is because I can use the
same browser on all three of my system types (with the same exact bookmarks file, UI, etc). For me, at least, it had less to do with IE being broken than FireFox offering me something IE didn't ... Cross-Platform compatibility. I would guess that I'm not the only one.
Also, I can see your point with iTunes not being broken, but it also isn't the same thing as MC. I know the widespread view of Mac users is that they typically aren't power-users, but I think that
especially when it comes to Media, that is a fallacy (Final Cut Pro, Adobe Photoshop, Macromedia Freehand, MediaPipe, and countless other OSX-available media-related programs certainly are not novice level).
There just isn't a program for OSX that does what MC does. MC and iTunes are no where near equivalent programs (and iPhoto makes me laugh). The closest you can get to MC's functionality on a Mac right now is probably Extensis Portfolio which is certainly in a different class with a price of $200 (and I would probably say MC is better than it). I see that as a reason TO go into the market, not to avoid it.
Plus, if you did go to the trouble of cross-platformizing MC for OSX, it would take only slightly more work to offer a Linux version too (since OSX is now built on UNIX).
Again, I can see the marketshare argument, but
who knows how long that will last...