"Stupid" may be an overstatement, but I don't think it's a reasonable argument. The more meat (or eggs, or milk, or whatever) you eat, the more misery you cause. Some animals are going to suffer because you're not a vegan that wouldn't have suffered otherwise. It may be a drop in the bucket of the whole situation, but it doesn't by far mean that the suffering of those individual animals that wouldn't have suffered otherwise doesn't matter.

It's sort of like saying, "I'm going to sexually molest my children because it happens so much anyway that the effect my behavior has is only a drop in the bucket." The main difference here is that you don't get to see the misery you're causing by eating meat/eggs/milk/whatever.

http://2.bp.blogspot.com/-ThndXIlb3iY/UD5OHBEmvdI/AAAAAAAAAlY/FV8kYCuf038/s1600/starfish+story.jpg

And this is to say nothing of whether we should be good citizens and do what it would be good if everyone did even if our own individual choices are of small consequence. It's like the "paradox of voting": https://en.wikipedia.org/wiki/Paradox_of_voting . Our own individual vote won't change the outcome of the election, so why bother trying? But then, what if everyone thought that way? Or worse, what if all the most rational people didn't vote for that reason? We'd be screwed!