I'm hearing teens say this all the time and it's something that "my" generation (I'm 32) never said. Is that even allowed? It makes the hair on my arms go up every time I hear it.
For example,
Teen 1 - "I think John totally has the hots for you and you should sooooo go out with him."
Teen 2 - "Shut up! He totally like not my type."
Teen 1 - "I'm just saying!"
*shiver*
Is it any use of "I'm just saying..." that you object to, or just the specific usage "I'm just saying!" (with the speaker not saying what she's saying)?
Just the specific usage "I'm just saying!"
I kinda like "I'm just saying!". It's a more casual way of saying "That is just one man's/woman's opinion". It's a little humorous and can be used to soften the blow of a harsh critique.
I actually don't hear it getting used in the way your example describes. THAT usage might annoy me!
P.S.
I say it and I am older than EekA!
I'm 29, and it doesn't strike me as odd at all to just clip it at "I'm just saying" without actually adding/restating your position. I take it sort of in a colloquial sense, dropping off the rest in a way of implying the rest of the phrase is self-apparent. As maybe one would say, "I'm not saying Mary is crazy, but if it walks like a duck and talks like a duck..." and leave it open to let the listener fill that in.
But I will say that there does seem to be a new trend of clipping off the ends of known phrases, which is beyond irritating to me: "they just did the whole nine," with no mention of "yards"; "he got mad out of the clear blue," with no "sky."