I feel as though I've heard this in films or TV shows from the 50s... but with an added "that's all" -"I'm just saying, that's all!" - in instances where advice or information was being given by someone and a negative reaction was anticipated or sometimes seen by the listener, so then the speaker quickly said that phrase as if to halt the reaction and sort of smooth it over. I distinctly recall it being used on Seinfeld and The Simpsons as well.

Personally, I dislike it's current usage or over-usage and see it as an insinuation. Whether that be insinuating something is true or false, insinuating that the person can't be held accountable for what was just said, or suggesting that the reader/listener draw their own conclusions based on the info, it's all insinuation to me. Insinuations by nature, are indirect statements - to me, a sign of one who may prefer to avoid confrontation/responsibility and not offend anyone. Whether you find this to be a positive or negative trait, is completely subjective, and for me, it's a case-by-case basis. I'm not big on generalizations and I think the intentions behind speech sometimes don't match what is being said.

There are those who'll throw in that kind of thing simply because they picked it up in their vocab. There are others who's humor is such that "just sayin'" fits in perfectly as a post-punchline to whatever they were attempting to make a joke of. That kind of usage of the phrase makes me think of the old saying "there's truth in jest." In finding humor in truth, you risk offending anyone living with that truth.

In reading comments here, it bothers me to think that some feel saying "just sayin" gives the message that "I don't want to discuss this" or "I'm not trying to fight." It's almost like using a silencer ("just sayin") at the end of a loaded gun (what was said before "just sayin"). If someone is broaches a topic with another, whether light-hearted or serious, that other person shouldn't feel compelled to not be able to discuss their opinion or thoughts as well. Maybe the fact that a little statement like "just sayin" can be as powerful as to 1) provoke this much interest 2) be believed by some to disable further comment on the subject by the listener/reader 3) be used as frequently as it is 3) understood to have both ill-intended and well-intended meanings; says something about the state of "Freedom of Speech" and maybe even the mentality of a large population in this country. As I said, I see "just sayin" no matter the intent, as a type of insinuation or implication, a way of saying something indirectly. Why would so many feel the need to be this indirect if in fact we felt comfortable, confident and secure with the speech we have the freedom to use, as well as with ourselves and others whom we choose to have conversations with? Is it our fears, whatever they may be, that hold us back from total direct expression of what's REALLY on our minds? THAT is what I'm saying, and it's totally up for discussion! =)