It depends on many factors. For a wedding reception, if you have a single DJ working alone, it will likely be very difficult for him or her to smoothly coordinate all the different aspects of their job. If all you want is for him to play music and make basic announcements, then a single DJ is probably acceptable. But if you want a true Master Of Ceremonies, then he needs to be free to move around the room, while still keeping a DJ/sound engineer in place at the DJ booth.

Gathering the wedding party for introductions, toasts, specialty dances and other activities can be a full-time job. Of course, you could always just have the DJ get on the microphone and announce, “Could we please have the wedding party come to the head table for the toast?” But that would get pretty old throughout your reception don't you think? Directing the details of your reception is better done off the microphone, quietly, and behind the scenes.