Ok i've read here that lots of people recommend not to get combo drives, but single for each function.
I don't think that a DVD burner and a DVD reader are really needed as the burner can read DVD's as well. I think they ment getting a CD burner and a DVD burner... I don't really know much about drives but I'd skip the DVD reader.
CD Burner -- No DVDBurner/DVDROM will ever match the CD functionality of any decent CD burner. That is, while there *are* two-sheep non-CDRW drives, they usually just glide in as bare minimum qualifications (basic EFM functionality, usually not beyond the capacity for SD2.7 .. considering current version is SD4.something its a bit back)
DVDROM -- Straight out of the box, DVDROMs own everyone for DVDripping speed; also, not beeing burner drives they DONT have the ability to detect ATIP (which has its uses, say with SecurROM backups). Furthermore, DVDripping is stressfull and better to put stress on a cheap drive designed to do just that
DVDRW -- Nothin to say really, DVDBurners give you DVD burning capability.