I think another way to save costs (if that is what the stores are after) is to make the cashiers they do employ more efficient. For instance, while living in Melbourne, Australia, I sometimes went to a local Aldi store and the cashiers there weren't exactly slow, but not the fastest either. In comparison, while living in Germany, if you go to a Aldi there, they check your items out faster than you can chuck them into your cart! (Yeah, that's right, they don't pack stuff up in bags for you over there.)
It's really amazing how fast they can be... and they have to because it's sometimes unbelievable how much stuff people in front of me can fit into a single cart. Because of that, however, you still get waiting lines -- it's the sheer mass of items people put on the conveyer belt. But the speed at which Aldi cashiers operate could certainly be a lesson to other stores as well if they want to maximize throughput per cashier.