I have one. I like it because a) I can circulate the water at levels below the skimmer if I need to (haven't needed to yet) and b) mostly it really cuts down on the amount of crud...I hardly ever have to vacume. Easy way is the kids make a whirlpool and I just use the pole brush to push any crud to the center, and the bottom drain sucks it all up. Works wonders.
You do have to be careful to winterize it properly if you are in an area prone to freezing temps. Also if installed incorrectly you can have real liner problems, I saw that on my SIL's pool - she has one too.
But I agree, it's certainly not a necessity.