I'm surprised how impractical the armor in fact is. In "Endgame", I perceived it as the ultimate defense, too.
Btw, that's also the way it appears in STO: Intrepid
-class (and their derivatives) vessels can activate the armor for about a minute. The ship is now almost indestructible, unless its hull was already severely compromised. You can still fire torpedoes but energy weapon systems go offline as long as the armor is active.
No successor to common shields then. At least that explains why 25th century (STO) Starfleet doesn't equip its entire fleet with an ablative armor generator. Plot hole resolved!