Depends on how one sets it up and what purpose. I use powerheads to get more ripple at the top surface. Without the extra ripple I was having a shiny film on the surface and I wanted to be sure of good gas exchange. Some use powerheads for extra water movement to keep debris moving so it will find it's way to the filtering. Powerheads can also power various filters from DIY built to standard sponge filters.
Not likely for mbuna if they have rocks to hide they will find a place out of the current but then there is no need to make them fight all day! You will also find that quiet spot will be where the grung lands as well.