It isn't more work to bring a dessert instead of more courses, but making the magic for all guests takes effort no matter how much they order. (And it can take much more effort, even with less food, if the guests are difficult.) We've overheard neighboring tables arguing about how little a tip they could get away with leaving -- maybe even no tip because they really didn't order very much! One table argued that if all the other tables left X amount, they could leave zero because the other tables would cover everything. And, of course, that was a table that ran the server ragged with nit-picky requests/demands!