From wikipedia it sounds like the advantage is not really speed of recharging but just that it will repeatedly fire for as long as the lever is turned without any other actions or pauses needed in between. Maybe not losing 10% (or whatever?) of the time on bolt feeding was sufficient advantage? Maybe the ease of operation in a hectic battle situation was advantage enough? Or maybe the continuous power requirement made it more feasible to use multiple soldiers at once working at higher speed, without them having to synchronize starting/stopping/waiting every x seconds?