I swear only when I'm trying to make an impact on someone with what I'm saying. When you curse when you are talking, it hits an emotional cord that you can't hit without swearing. Marilyn Manson actually is a perfect example of this... He swears alot in his music but rarely when he is speaking. He's swearing in his music when he is trying to deliver the message. Sometimes, swearing can make the message you are sending across stronger than if you were to not swear.
Also, when someone swears when they talk, it makes them appear more trust worthy. (sounds crazy... I know. But its true) This doesn't mean that you should swear like a sailor... But it's not a bad thing to drop an F-bomb every so often. As long as you don't sacrifice sounding intelligent by sounding like you live in a trailer.
If someone doesn't swear AT ALL when they talk, I usually don't trust them. It usually means that they are trying to sensor themselves (which they are if they are actually consciously not swearing) and I don't like when people try to be something that they are not. Its fake... And if you want girls to like you, the best thing to do is to be a REAL motherfucker, motherfuckers
Another thing... Not swearing when you talk makes you come off as gay to others. I've had students I've worked with who really we're "good boys" and people would think that they were gay.