I run a 56g without a sump, and it's working out well. I am not using a ton of equipment though, and I am starting to look at some gear that may present some problems. I want to add an auto top off, but there really aren't any good options for a tank with a rim like mine. I can see where having a sump would be a plus here. As well, I am getting a GFO reactor in the mail Tuesday, and I am debating mounting it in the stand or running it HOB. Again, It’s just more gear in the tank, so I can get away with it now, but the tank is starting to have quite a bit of stuff on it. Plus I would love to get my heater out of the tank.
I guess I’m saying that you don’t have to have a sump if you want to keep your tank lean and mean. I don’t use one and have a very healthy system, but I would imagine that they are kind of awesome if you are going to run a bunch of gear.