I was at a client's house today and he had a pretty big pitbull. My guess is that it probably weighed at least 50 lbs... but it was pure muscle. Definitely pretty scary looking.
He said the got the dog because his house was broken into twice. He hasn't had a break-in since getting the pit. So that's one reason to get a pit. Obviously there are other breeds that also work well as guard dogs (or at least as a deterrent)... but most of those breeds are pretty big. Not everyone wants to live with a 100 pound animal. There's no other 30 pound dog that would be any use as a guard dog.
That being said, I would never want to own a pit. You can basically never take them off leash in public. You can never leave them alone with another animals (at least you shouldn't if you care for that other animal). You should never leave a pit alone with children (and probably shouldn't even have children in the same house at a pit). As an owner, you have to be constantly on guard because a lot of these dogs are ticking time bombs and you have to constantly be aware that they can be set off at any time for seemingly no reason.
Depending on where you live and what insurance company you use... home insurance can be a real issue if you have a pit:
http://thelawdictionary.org/article/...surance-rates/
There was a discussion about getting a pit in the german shepherd forums for anyone who is interested:
http://www.germanshepherds.com/forum...out-there.html
Basically, a guy who just got a female german shepherd puppy also recently rescued a young female pit. Almost everyone in the forums was trying to tell the person that it was a horrible idea (because female GSDs can be very aggressive towards other female dogs... and pits are just aggressive to other dogs in general). Many people think that these two dogs will have to be kept apart at all times with a "crate and rotate" program (where one dog has to be crated while the other one is un-crated).
Obviously the forum is somewhat biased towards german shepherds... but that forum is filled with a lot of knowledgeable dog people... and german shepherds are obviously a big dog with some problems of their own... but even these people (who are very dog-friendly) didn't have many nice things to say about pits.