This is doubly true if you're stranded on the side of the road in the middle of snow storm. I once had to ask the 911 operator to send someone, I didn't care who. That is just a recipe for disaster. You don't know if these people are qualified, they may damage your car, charge you way to much, etc.
When you get a AAA contractor, you know you are getting someone that AAA has checked out. These people get a lot of business from AAA and they want to make sure those customers are taken care of. The last thing they want is to irritate their best customers and get a negative review. I am sure those contracts are updated regularly and no one wants to lose a AAA contract.
My experiences with AAA contractors have been nothing but positive. They have been very professional and courteous even going as far to offer me some free coffee and donuts. They are even willing to skirt the rules occasionally, but I am sure every contractor is different when it comes to that.
What it comes down to is quality and AAA service is the best. I've had to be helped several times and it's never easy. I'm not always in a good mood and more than a few times I used that 100 radius limit to the max.