I'm not a costumer myself, but I think costumes add to the fun in the world. I'm disappointed by conventions that don't have much hall costuming, and I think the world would be a better place if, say, 10% of people on the street were costumed. (Up to fan standards, not cheesy Halloween costumes.)
I suspect it's a matter of people who are so afraid that mundane personas are so unsatisfying that if people were allowed to try anything else, they'll never go back, but this is only a guess.
Is there comparable prejudice against cosplay at anime conventions?