With easy access to music thanks to file sharing on the Internet and the ability to rent a PA system for a few hundred dollars, it seems as though it might be a good idea to just have cousin Jimmy bring out his laptop and act as the DJ for the wedding. That would definitely save a few bucks. Besides Jimmy really knows his music. Well, in certain situations this might work out, but not using an experienced professional wedding DJ can be very risky.
I have worked as a DJ at well over 500 weddings in the Edmonton area over the last 14 years, and I have been a guest at many weddings were I've had the opportunity to observe other DJ's (professionals and amateurs) at work. Having a professional who really knows how to get people on the dance floor and keep them there will make or break the dance portion of your wedding reception.
More times than not I have seen the amateurs have a really hard time. First of all, no matter haw much music they've downloaded, they just didn't have the selection available to respond to all the musical tastes off the guests and to expand on the music that's really working with a given crowd.
I've also seen amateurs having technical difficulties almost every time. They are using rented equipment that they are not familiar with and generally don't get it hooked up correctly. If they do get that figured out they then have challenges using it all properly.
I think the main reason that a non-professional will fail is that the typical wedding will have a very diverse range of guests varying in age from children to great-grandparents, with musical preferences to match. It takes a lot of learning and experience to figure out what all these different guests will respond positively to and how to keep the balance between them all just right. So even though cousin Jimmy might really know his music it is likely that some factions of the audience will feel he his neglecting them.
I see many couples putting so much thought and planning (and money) into to the catering, decorations, wedding cake, dresses/tuxedos and photography but then trying to cut corners and save money on the dance portion of their reception. A typical wedding dance usually takes up to four hours or more of the reception. This is by far the biggest part of the whole reception, so why neglect it and leave it up to chance with an amateur? Go Pro is my advice.