I agree with Dwayne. Another instance would be if it were a tour that you had your heart set on doing. For example, I booked a couple of shore excursions today for our cruise in July as we would be disappointed if we were shut out.
Does Carnival charge the tours to your credit card upon booking them, or apply the cost to the shipboard account? HAL charges for them when you make the tour reservation, so my Amex got a bit of a workout today. That would be another con, in my book.