If you plan to do any dvd copying, then keeping the dvdrom can be a good thing.
For example, with a single drive setup, you'll have to put the source disc in the burner, hang around while it rips the dvd, then put a blank dvd in for it to burn to. W/ a dual setup, put the source in the dvdrom and the blank in the burner and walk away.
Other than that, and the occasional cd copying, a single drive would be fine.