hm.. You're dealing with someone who usually wouldn't get the cake, but I could see the difference being "want. out. now." vs "hey we're relaxed and we've got time so we might as well stay for that last course."

Like Sparr is perhaps saying (I'll admit I haven't really been able to parse quite what he's saying) the $1.20 tip on the $6 cake doesn't really come into the decision. If I really like the server I may add more to the cakeless tip, but I won't take on the extra $6 (and 600kcal) cake out of conscious loyalty to the server.

But I'm willing to believe that comfort/pleasant experience might lead someone more cake-ambivalent to order the cake.