Then maybe we will have less pissed off professionals, and less unhappy events in our lives. I've heard of a few instances where the worst case scenario happened, and I know that for myself, I'm happy with declining every offer that comes my way, paid or not.
Once you start saying yes, it's very hard to stop saying no, and I think I should be more responsible than that. It isn't just about doing favours, nor about honing your skills. Sometimes, especially for weddings, the couple only gets one chance, YOU only get one chance, are you confident enough to ensure that you will produce something good enough for them?