Many people prefer a dedicated reader simply because DVD-ROM drives are faster (they read DVD's faster then a DVD burner). If you plan on having the 851 for a very long period of time (more than 2 or 3 years) you might consider getting a DVD-ROM drive simply to extend the life of your burner.
So, overall it's good idea but not a necessity (for example I don't have a DVD-ROM drive, I'm not in a hurry so my 811S reads fast enough for my needs and I plan on recycling drives every year or two so I'm not worried about life span either)
If you decide to get one, the LiteOn DVD-ROM drives are top notch. You can get a 166 or 167 for pretty darn cheap. For example the 166 is only $30 at newegg.