I don't think a MTBF would really make sense in a DVD burner context. It's not doing anything unless I tell it to read a disk or write to a disk.
I could have it on for 10 years and never tell it to burn a disk or read a disk. So if you want to have a meaningful MTBF, you have to define some statistics for how often people use their DVD drive. Where do you get those from? If you're too conservative, people will accuse you of trying to pad the statistics. If you are too liberal, then your drive looks bad.
So in my mind, MTBF doesn't make sense for a DVD drive.
I think the main things you have to consider turning it off for are mechanical like your CPU fan (unless it's only a heatsink), your case fans, your power supply fan(s), and your hard drives. All of those are mechanical parts that will wear out and are in constant use.
Having said that, I never turn my computers off. Once I bother setting up software suspend, I might consider it for everything but my core servers.