We use propane for convenience and ease of cleanup. We use the grill all the time while camping... breakfast, lunch, and dinner. Also to warm up leftovers at any time if people are hungry. When we are with a large group being able to start or stop propane grills instantly is great for organizing eating times, and keeping food warm while serving and for seconds.
Is charcoal really that much less expensive? On our portable propane grill, we use the little 1 pound propane tanks, and one of those will last multiple weekends. I am not sure how many "cookings" exactly, but it seems like when we've used charcoal we go through a bag of charcoal much sooner than a one pound tank of propane.
The 20 pound tank that I use on my home grill will last months of constant use. Friends that use charcoal a lot seem to be always needing to buy a bag of charcoal. And yes, we've run out of propane at inconvenient times.