about the only use for them is being able to shut off part of the map. you can split teams up quite effectively or keeping someone trapped behind them while you recover your shields.
Have not noticed any real use either.
Are the Promethean grenades useful ?
I think they suck your shields but why would you stand in one. Not instant is it ?
Still playing campaign but in all the fights I've haven't noticed them doing anything but that might just be the carnage hiding the clues.