Think about it: as others have pointed out, THERE'S NO DOWNSIDE TO RAISING SHIELDS.
As far as we know.
Yet we do know that starships virtually never raise shields. In a couple of TOS and early TNG episodes, there's an auto-raise function when the ship suddenly encounters unknowns - but when Kirk or Picard encounters an unknown let alone a known while having time to think about it, the shields stay down.
This is confirmed basically everywhere in Trek - from the very beginning ("The Cage" and "Where No Man Has Gone Before") to the most recent incarnations (part of STXI hinges on it being unheard of to raise shields "just in case").
So there probably
are serious downsides to raising shields. Not just keeping them up for great lengths of time, mind you, but raising them at all.
Some possibilities have already been mentioned. Raising shields could create a "diplomatic incident", i.e. make the opponent fire at you in justified preemptive self-defense. Or it could block your sensors or communications devices, making you miss a vitally important piece of information. Or perhaps it would tell the enemy too much about you, giving him time to evaluate your defenses before acting? It's also possible that shields are more delicate than we think, and only give a limited number of raisings between overhauls...
In any case, Kirk did nothing unusual in ST2 regarding shields. Doesn't mean he couldn't be prosecuted on
that, of course. But it would go against well-established Starfleet precedent for Kirk to act, and it would be a bit unfair to prosecute him for following the book. At the very least, the writers of the book ought to stand accused as well in that case.
Timo Saloniemi