>Would you agree, though, that customers wouldn't simply enter the shortest available line?

I don't agree with that at all. In reality, some would and some wouldn't do what you suggest. If all of them did exactly what you are suggesting ( some sort of extreme rational agent), the distribution wouldn't be exponential. But then, all human beings don't buy low & sell high, nor do all of us withdraw money from the same ATM at the same time, nor do we make phone calls at the same time, or hit the internet at the same time. Aggregate behavior distributions modelling aggregate behavior of individual humans work well except in very extreme conditions such as 911 when everybody does indeed make the phone call at the same time thus jamming up the airwaves and killing the shape of the nice Erlang distribution at the telephone exchange.