As long as you don't treat all kilts as costumes I don't think there should be any reason to object.

Personally, I think anything can be a costume if you are creative with it. Just show up in your regular clothes and go as a sociopath. I also enjoy the piece of paper taped to the shirt that says "world's laziest costume." And I once wore a bathrobe, tin foil hat and slippers (and went 2 weeks without shaving) to go as a paranoid schizophrenic. In all these cases the clothes were part of a costume, but do not constitute costumes without the explanation.

And of course it can be a good way for people to get more comfortable wearing a kilt in public, and around friends who might not be used to such a radical change.

Hell, wearing my improvised belted plaid thingy as part of a braveheart costume was what put the idea in my head to get a real kilt.

And right now I'm trying to figure out what to wear for the upcoming Halloween party, and all I know is I want to work a kilt into the costume.