It depends on why you're voting to close and whether you think the community as a whole will agree or disagree but there are plenty of situations where I don't see a problem with it.
If you're absolutely certain that a question is off topic and that it's going to end up getting closed then yes, answering is bad form but I'm not sure these are the situations we're really talking about.
After all in these instances even the rep argument is pretty weak. Bad questions don't stay open long enough to get the traffic for any serious voting and if you can vote to close your rep is already high enough that why would you waste your time putting together an answer for the sake of another 20 or 30 points?
But more to the point, there are also questions I've voted to close where I've done so unsure as to whether the community as a whole will agree. That's not to say I'm wrong in my vote or lack conviction, just that I don't think the way I see it is the same way that everyone else does. As often as not in these cases the question gets a couple of close votes and stays open.
At that point not engaging with the question seems an odd thing to do, basically saying even though you think you have something useful to add you're not going to because you think that everyone else has made the wrong call.
Now if that's how you feel then fine, that's your right, but I'm guessing most people would accept the majority view and engage on that basis. After all, if whichever party we vote for loses an election, so-one suggests disengaging with the political process as a constructive or useful thing to do - you get in there and make the best of the situation.
The other thing that I'd see supporting this course of action is that whether mediocre questions go on to be useful or not often depends on the first few answers which set the tone for what follows.
If you can help steer the mood of what you see as a borderline question in the right direction with a great answer, isn't that a good thing?
I do get what Jeff says about how this can be seen, but I tend to view it that if I'm honest, helpful and consistent in the way I behave then I'm happy for the community to judge me how they will and believe that on balance they'll be fine with it.