Two large Australian department stores implemented single global queueing for non-self-serve when they introduced self-serve. Its a mixed bag because they also reduced staff on the line to match maximal delay for minimum fuss: ie, they don't aim to serve this line any faster than the worst-case single line instance.

It doesn't invalidate the single global line theory: it points out that the qui bono moment in a queue is not you the customer: its the profit centre. They do anything to maximise profit absent a strong signal they went too far.

its morally akin to basic time-and-motion/taylorism.