Go Combo
Combo Drive, no question at all.
I'll have to disagree with a lot of the posters in this thread.
I have two SuperDrives, I got my first one on my iMac (my only computer at the time), and my second just recently for my PowerMac.
I originally thought that I'd use the drive to burn movies or do backup. But that didn't happen because of a couple of things. The single layer drive doesn't allow you to backup a DVD movie (one that you already own of course) without a lot of work, double layer would. 4.7 GB DVDs for backup aren't very helpful, it's too small, and really only the latest one or two backups are worth anything. After a couple of months you'll have a stack of expensive DVD-Rs that are now useless.
Save the ~$200 and get the combo drive.
If you find a need for DVD burning, you can always use part of the $200 you saved to buy an external Dual Layer burner.
If backups are a concern, get an external HD, it'll give you more capacity for large (5+ GB) backups, and you won't be littered with piles of media. For VERY important backups, get a 1-2GB flash drive and keep it in your freezer (in case of fire).