Unless your burning DVDs (and who actually does, not THAT many), CDs are perfectly fine for most people, as almost everyone nowadays has a large-capacity backup device (mine being a d2 LaCie 250 GB hard drive). If for whatever reason they need to, buying an external burner would be just as effective.
Lots of people burn DVD movies, especially people with children, nephews, nieces, or grandchildren.
Considering that a good bit of software is now published on DVD's, a DVD burner is essential for making backup copies (in case the orignal gets scratched, lost, etc.)
Also, if you do any mapping or large graphics work, a DVD is often the best way to send media files to a client.
The only thing that I don't use DVD's for is backing up my system. That's all done on my desktop with redundant drives.
Finally, I want my laptop to do everything when I am on the go. Carrying around an external DVD burner is as cumbersome as carrying around another laptop.