It is ok, who needs more power while on trip or something.
But it is not healthy for the phone battery to be charged like this. The difference between battery and charger is battery has constant voltage, while charger has spikes (constantly adjusting voltage up and down) that "push" the charge into the battery. Those spikes can be heard as a whistle coming out of new smaller chargers. If a battery is charged with a constant voltage (with another battery) it is never fully charged and charging is slower.
So, you won't harm your battery if you use this device while its battery is full, because phone then stops charging the battery and uses outer supply. You also won't harm it if you use it rarely. But constant use on empty battery will for certainly damage it, shrinking its capacity and shortening its lifespan.
So if you use it, use it only when you have to, try minimizing the usage of the phone while it charges through this device, and after usage charge your phone regularly, via charger.