Well, yeah. Whats to stop the player from doing it is that, last i checked, forcefields trigger a 'all ai ships respond' threat of some sort.
Additionally, they are incredibly slow, cant use wormholes, and still suffer from the same stacking issues as before.
In effect, the only difference this would make as far as using them in a fleet goes, the only change is that it takes 4 times shorter to walk over stuff with a forcefieldblob than before.. The forcefields still cant be repaired while taking damage (pretty much the largest exploit regarding forcefields was fixed).. Which means its not impenetrable, its merely difficult to break. And the AI excels at throwing around brute force.
And turrets would still get the 25% reduction in damage, and those are kinda a huge part of defense, under forcefields or not.