A common term that professional wedding photographers
The problem with "Uncle Bob" is that, while he or she probably takes very nice pictures of flowers in the garden, the family dog, and occasionally a good family photo, "Uncle Bob" is just not experienced at photographing weddings.
Weddings are a "live" event where just about anything can and will happen to throw a wrench into the schedule. A professional wedding photographer knows this and can adapt to situations that arise with a transparency and skill that can only come from experience and training. "Uncle Bob" just doesn't have anywhere near the experience, skill, and training of a professional wedding photographer.
A wedding... YOUR WEDDING, is a pretty important event. Some would say a VERY important event, perhaps one of the most important events in your life. When I see people recommending having a relative or friend photograph a couple's wedding, I cringe. Not only is this putting a HUGE amount of responsibility on a person who is unqualified to do the job (and likely also isn't even aware of just how unqualified they are until the day of the wedding), but it's setting up the bride and groom for some serious disappointment.
A wedding day isn't a repeatable event. Unlike a portrait session or a commercial shoot, where we can re-schedule if there is a problem or the shots don't turn out as the client wanted, you can't do this with your wedding. You could try, I guess, to get all those guests back on a given day, at the same location, with all the same attire, same decorations, etc... (you get my point), but I don't think you'd be very successful, nor would you want to.
When all is said and done, the only thing you really have left to remember your big day with is your wedding photos. You've put a lot of time and effort, not to mention expense, into such a big event, so don't you want to make sure your wedding photos will be beautiful memories of that wonderful day?
I feel bad when I hear stories of couples who were very disappointed with their wedding photos taken by "Uncle Bob". I have couples contact me hoping that I can "fix" their photos that "Uncle Bob" took, and I feel bad when I have to tell them that there is very little that can be done. Even when something can be done, the amount of time involved in "fixing" their "Uncle Bob" photos is going to make it a much more expensive process than if they had just hired a professional wedding photographer in the first place.
Take the time to look through the portfolios of professional wedding photographers. You'll quickly see the differences in style and vision, but more importantly, you'll see the difference in quality. Compare the images of some of the best professional wedding photographers with those shot by someones "Uncle Bob". The differences become VERY evident, VERY fast! You'll quickly develop a feeling for what you like and what appeals to you, and you'll be able to see why having "Uncle Bob" or a "bargain" photographer shoot your wedding is just not worth it... at any price.