Yeah maps have pretty much no cover and the few cover spots aren't in the best tactical positions...
I do question this suggestion however.
How would we know they are holding a shield? There is no visual indication, and this seems like a big enough deal to require it. Not everyone has awareness, and even then awareness is good more for skirmishing and focus fire, not determining who is holding a shield in the middle of a chaotic clash between two large forces.
What exactly would the statistics of this shield be? Can you block damage from everything, or everything except explosives? etc.
I think it's best to wait for devs to properly implement covers into the map.