I've seen that as well, I think they do it to show off some kind of flashy beltbuckle or something. I think it's some kind of new trend or fashion statement. Ofcourse, I only see preppy kids or wiggers as some would call it, wear them.
If i have a vest over it, I sometimes don't tuck it in. . .mainly because my shirts aren't usually that long.
"I wasn't getting rid of you. I want you to do whatever you want to do with me. I know that sounded dirty, and dirty things count, but I didn't mean the dirty things. You and me can hang whenever, wherever." -Lorelai Gilmore
I hate tucking my shirt in. I had to do it in school... I'm fat and it just looks stupid. And I was heavier in high school than I am now. I used to wear my sweatshirt over my regular shirt even in the summer to hide it.