How common is it to see guys wearing kilts in Scotland?

Not at all. Unless you're at a wedding. Kilts are very expensive to buy so most men don't have one and just hire them when they're needed. It's just one of those weird stereotypes. We don't all eat haggis. We don't all play the bagpipes. We don't all have special connections with the Loch Ness Monster. We don't all play golf. And we don't all wear kilts.