Yeah once shields are out of the picture, it doesn't matter what the matchup is,really... It's just another structure that can be destroyed with a few torpedoes. Of course there are exceptions, like a borg cube or something, but I think a galaxy class without shields is just as vulnerable as anything else.
Of course, it's still stupid for riker or geordi or data not to try and just change shield frequency.... again, bad writing.
I always figured they just didn't want to overwhelm the movie audiences with unnecessary technobabble. It stands to reason if they tried, Geordi would be the one doing this, or at the very least would be looking now and again at the monitor with the new frequency, and the Klingons would adjust their weapons appropriately.