You are correct the pump is used the most at the start of the siphon, the problem is that mine several times failed when it restarted. The little pumps do pull the air out of the tube but as with the U tube you dont need a mechanical pump at all. Its design is better in that it will clear those little bubbles that build up on its own much more effeciently. The other big perk is you dont need to hook up one or two pumps to make sure it doesnt fail, just a waste of electricity imho, and the model I had even with a pump attached it often failed to restart the siphon because the rear chamber was too small and couldnt pull enough water up to reactivate the siphon.
CPR's arent horrible but everyone I have known that has ran them any length of time has had them fail, the lifereefs, not a single person I personally know has had one fail. CPR weakest points are at the start of the siphon, so when the power goes out is when I always had trouble with it. You can also think of it this way, the CPR may be cheaper to buy up front but when u factor in the electricity it takes to run, and the replacement of the pump over a time spand it ends up costing more.