My take on it is..
Open/tweener cockpit ASS is a more "normal" position to start out in, more "chopper" than anything else. I find it pretty useful on the open road at speeds past those on the race track (ie down big hills) as I've always found it a bit easier to hold a line. Also gives options for different hand positions.
Tiller/hamster/closed ASS is pretty different to what people are used to, it's worth an extra mph (according to a certain designer from Norfolk) at race speeds. Once you get used to the position it can be very comfortable, also looks cool/even more out there, to the non-cyclist.
USS can take longer to learn to use as there's no correlation with an upright, with ASS you at least see the bar in front of you, which is something people seem to find helps them initially. USS is great for touring (yes I know cliche) as it allows you a more uninterrupted view, plus the benefits the others have said.
I remember vividly the first time I went down a very steep hill on my old SWB and was convinced I was going to fall out of the front (managed it off the back a couple of times climbing though)..
I've got 'em all, my preference depends on my mood, but I still don't like ASS on trikes