Some places do this. Fry's Electronics, for example. It works fine. Some supermarkets I've been in do this for their express lines ("10 items or less").
Oh, and airline ticket counters are routinely run this way.
Add to this, Fry's also stacks the "wait line" area with lots of high-margin low-cost goodies like candies and trinkets. I myself have succumbed to grabbing some trail mix or a thumbdrive while waiting there.