Probably nothing. But very few people who aren’t hardcore computer users will buy and install new hardware whether they need it or not. Most folks will just wait until they buy a whole new machine. That said, there is no reason NOT to buy a machine with a DVD drive these days. Most new machines–even cheap ones–come with one by default. So why not have one? They play CDs too, and they aren’t any more expensive than a CD-ROM drive.
I got my first DVD drive when I bought my last computer, in 2000. At the time, I noticed that it a good DVD drive cost the same as a good CD drive, so I said “why not?” I really don’t understand why everyone doesn’t have one at this point.