It's not so much the turning them off and on, it's doing so repeatedly (BTW questions NEVER bother me so ask away!).
Prop pumps have problems starting in the correct way unlike impellar pumps (impellars can spin either way, props only work one way). To make them spin the correct way, resistance is applied to the shaft (utilizing gravity) and the prop automatically corrects itself and spins the correct way. The early Tunze nanostreams are having this exact problem. The shaft is too loose on the pump and the lack of resistance causes them to spin backwards indefinitely or until you put something near the pump's output (now intake as the prob is going backwards). The suction causes resistance, and the prop corrects itself.
This problem with them starting makes prop pumps (nanostream, hydor, maxijet mods) unable to be used on a wavemaker. The normal on/off is expected but to do so continuously will ruin the pump.
All the big tunze streams have a braking system that forces the impellar to always spin the correct way PLUS the streams never power completely down, only to 30%. This also happens to be the reason you can't clean a stream with muriatic acid, the brakes will melt.
John