Sounds like a potential startup. :) If you could build a better one (which shouldn't be that hard if they're so bad, right?) you could dominate!
I like it when they have both, since sometimes I want a checker and bagger, especially if I have a bunch of stuff. Other times I just want to get in and out and the machine is consistently faster than checker + line.
Of course, the real issue is that lines are good for businesses. They keep the customer in the store longer, with more opportunity to spontaneously buy all that shiny stuff near or at the checkout stands... WalMart has like 10 self-checkout machines but every time I've been there they've had most or all of them turned off. I don't think it's because all of them had errors.