I see this more and more, that a 60+ player zerg that wants to take down a t2 or t3 tower/keep will layer their rams / catapults / trebuchets with shields and attack without worry from any defensive siege. Only a zerg can counter this type of play style now, which is frustrating for small groups trying to defend.
Defensive siege is essentially useless. Catapults, arrow carts, ballista, trebs, and siege disablers are all blocked by shield gens. That's assuming that the attacking zerg has not aoe'd your wall defenses down to nothing.
Even with Inulnvurable Fortifications, it's often not enough for a counter zerg from another map to save a tower against the multiple shield + 5-6 superior ram combos, especially with a keep waypoint contested.
The current method of using overwhelming siege makes defense very limited without a sufficient counter zerg to come to the rescue. You might as well just turn each tower into a PvP point that can be captured... the Lord ring, without a defensive barriers.