I’m from a poor section of rural Idaho and it’s common enough there that I picked it up and use it in casual conversation. I also say things like “I ain’t” and “I got.” I don’t use it in business coversation (although I did slip up the other day and say I’d get to something if “I got enough time”) but in talking to friends and my wife that’s how I talk. It’s not a matter of being uneducation (I have a college education and am fairly well read, if I do say so myself). It’s probably either a rural or lower-class speech pattern.
I know people don’t like it when I say this, but there’s no reason to expect people who are highly educated to necessarily speak Standard English. Standard English is certainly a dialect that tends to be associated with education - it’s the prestige dialect, which means that Standard English is socially associated with the upper class and to some extent, it’s explicitly taught in schools. Most written English - and all academic writing - is in Standard English. But those things are all connected with the variety’s social situation - none of this reflects some inherent quality of that particular dialect. There’s no reason why an educated speaker of English should particularly be expected to speak exclusively in Standard English (as the experiences discussed in this thread illustrate.) Some people make an attempt to lose the particular regional flavor of English that they grew up with. It’s probably a good idea, simply in order to avoid being judged by ignorant people who are going to make (clearly) unwarranted assumptions about your education based on the way you speak. But the fact that some people don’t try to give up their native dialect even though they’re well-educated shouldn’t come as a shock.
I have to agree with you, Excalibre. Mostly because, as a Chicagoan, I cannot seem to train myself not to put a preposition at the end of a sentence. (“I’m going to the store, want to come with?” “Where’s it at?”) 
“I seen” is also common in uneducated Australian English, but even there it’s not exactly standard, and it’s used by some (particularly rural) people who don’t fit into that demographic, but that’s a little bit rarer.
Generally, it’s not common, but it doesn’t raise eyebrows either, most of the time.
I know. I was just being silly. Maybe that would have been clearer if I had misquoted the whole chorus instead of that one line. In fact, that’s what I did the first time I tried to post in this thread, but the board had just ground to a halt. I just didn’t feel like looking up a lyrics website again so I could copy and paste.