Having been a mobile DJ for over 15 years, I can tell you that JUST playing music ignores the primary reason to have a good DJ; Master of Ceremonies. The primary role of the DJ at the wedding is to coordinate the events, the other contractors and the guests, so that there is a smooth flow of activities, and photographers have their cameras prepped in advance of the special moments.
When I got married over 15 years ago, we justified the purchase of a LaserWriter II by producing all of our invitations on the Mac. We used pre-printed 8 inch square cards and envelopes, that had appropriate color designs around the border of the card; the invitation verbiage went in the center. We also included another 8 inch square card with additional information; times, directions, gift registry info, attire, etc.
Last point, by using a database for keeping track of guests and gifts, including showers, we printed the RSVP cards like a merge document; each card had the name of each guest. How many times have guests sent in the generic cards, checked they would be in attendance, without writing in their names?
Mac Your Wedding