Variations of this argument have been made a number of times in this thread and elsewhere, and they often come up whenever anyone finds something worth devoting their time to.
But really, are you as strictly utilitarian about everything in life? This thing that person X is doing isn't worthwhile, do something else!
I mean, how many threads and posts on this very forum are actually worth the effort we put into it? I mean, if we want to change people's minds, or promote critical thinking, shouldn't we be doing something else?
It seems to me that this is basically an informal fallacy that I don't know the name for, the "Do Something Better With Your Time Fallacy". It isn't related to the argument itself, only that the effort of making it isn't worth it.