So, honest question, would you not trade the internal CD-Rom for better battery life, more ram, lighter weight or some other tangible advantage?
The only situation where I could imagine an external CD-Rom to get in the way would be for watching DVDs on the go. But afaik even for that most people copy the movie to the harddrive beforehand - to prevent the drive from sucking the battery dry while watching.
Instead of the above trade off, I think I'd buy two machines if needed.
I see the MBP as a laptop to use plugged in a socket and with a lan cable at home, work, a client's place or a library, and only need decent portability and battery life. For all other uses I'd prefer the ipad or the Air(wish I get the $$ for it).
I see where you're standing. But actually I receive two DVDs per week in my mailbox, so last time was today. And I still buy CDs, yes.
I think what you really want is the next iteration of the Air with whatnot is missing from the current model.