Situps aren't useless even if spot-reduction doesn't work. If for example you used to be a fatass like I was sort of and you put on a ton of visceral fat, which stretched your abdominal muscles esp. the transverse abdominis, you could return to your original weight with more muscle than when you started (less fat overall) and have a much bigger gut because the muscle is stretched out.
Anyway, working the abs can fix that and it's a pretty good idea to have strong abdominal muscles no matter what you do.