Given the option between booking a great wedding with someone who isn't sure that they really need your services and booking an okay wedding with someone who will appreciate you to the fullest extent, the "okay" will always get my vote.
I have the opportunity to decide what clients really need and want, when we sit down for our initial consultation. During this time I can figure out how much they REALLY need a planner, how much they will understand the services of a planner and whether or not they would actually be a good fit with my planning style. Most of the time they are a perfect fit, sometimes though, they aren't.
Financially, it certainly doesn't make sense to "turn down" business. However, as a coordinator who is interested in working in the industry, long-term, I think it is always wise to realize your strengths and weaknesses.
I recently received a lead for a traditional Indian wedding. While I would be thrilled to work with this event and to learn more about the Indian culture, I didn't think it would be fair for the bride to have to "teach" me along the way. I made her aware that I wasn't totally familiar with the traditions of the wedding, but I would be willing to learn all of them if she was so inclined to book me. If not, I could get her in touch with an actual Indian coordinator. Well, needless to say, she was absolutely interested in working with someone who really would "get" what she needed on her special day and I sent her along her way.
Did I just lose a large sum of money, sure. A potential client, yes. However, in the long run, I think I made the smart decision based more on her needs for that day, than on my bank account. She emailed me a few days later to tell me that she was thrilled with her "new" planner and that she was grateful that I got them in touch. While that won't pay the bills, it does make, what I do, seem totally worth it.