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).
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.