Our friends from WeddingWire hit the nail on the head with this. Thought you'd appreciate it! (We edited a little.)

HIRING YOUR FRIENDS?  BAD IDEA. Friends and relatives may offer to gift you their “services” as their wedding present. But just because Aunt Edna loves to bake and your 2nd grade BFF takes great photos on her iPhone doesn’t mean they can create what you’re envisioning for your wedding.

If they are a professional vendor? That's different. But experienced wedding vendors will think of everything you’ll need on your wedding day, that a friend with a hobby may not. (So you don't offend them: Tell your friend/relative you’d rather have them enjoy themselves at the wedding.)