I’ve performed a lot of weddings, but I’ve also been a guest at weddings, some performed by professionals and some not. The non-professional officiants are usually friends of the couple. They know them well and understand what sort of wedding ceremony the couple wants. Some of the ceremonies are terrific. Some are, well, lacking. They can be mumbled, incoherent, uncreative.
Professional officiants usually don’t mumble and they are able to deal with unexpected events, but they can also be incoherent and uncreative.
The trick is to find the right person for you – someone who will involve you in planning the ceremony, someone at ease in front of strangers, and someone who is responsible enough to file the paperwork.
Here’s an article from the New York Times that discusses some of these issues.