The problem with facing shields is that engineering adds so much resilience,
that this would result in even more broken combat.
Very much this.
And on the idea of having this as multi-crew feature: while it would be a logical thing to do for a co-pilot and would make sense, it lacks when you look the complete package: now you'd have a player along to do nothing else but move shields around? What else would he have to do? Even if you have more ideas of things he could do, how do you avoid it being just a random collection of minigames? How do you make a complete package out of it, which provides interesting and rewarding gameplay?
Over the years i've seen more than such minigame for a co-pilot being suggested and in other games even implemented. But it never added up to a complete role and a rewarding experience. And as long as it doesn't do that, it's not worth implementing.