If you have ever used that phrase it is probably for one of two reasons. Either you have said something shocking or offensive and need to recover, or you have said something that you want to hide your true motivations for. Let me give a couple of examples.
Shocking or offensive:
"That guy is so annoying that someone should cut off his favorite body part, burn it, then scatter the ashes on the ground and have a dog pee on it...I'm just sayin'."
You start a statement, take it a little too far, then realize everyone is staring at you with raised eyebrows and nervous smiles. That's when I'm just sayin' comes in handy. Throw out that statement and suddenly all offensiveness is removed from the sentence and eveyone can laugh along with you. Because you wouldn't REALLY do something if you're just sayin'.
Hiding true motivations:
"I know you're married, but we should totally go out...I'm just sayin'."
What you are really trying to say is "I want to go out with you and I could care less about the person you love and have sworn to be faithful to". With I'm just sayin', you have now made the sentence void and ended up contributing exactly nothing to the conversation. It's like using too much toilet paper, you don't seem to care, but then someone else always gets stuck with an empty roll.
I have exposed the truth. So to all you users of that phrase out there, we're not going to fall for it.
I'm just sayin'.