It's all about looking cool I think. When I've gotten blowjobs from chicks with tongue studs, it really didn't feel any better or worse than a normal blowjob. But of course, I'm a shitty person to ask because I don't even like blowjobs that much.
Kissing, though, I find to be more fun when the girl has a tongue stud. I like to play with it with my tongue.