@ is not short for "attention"; that would be "Attn:". I truly doubt that any memos in the company were addressed @ Lee. It would be Attn: Lee.
Until it came into use in email addresses (where it does mean "at"), the @ symbol was used in pricing and counting things, in sales. I would order 25 widgets @ $1.25 each, and that is how it would be written on the order form and invoice. So it is more like a mathmatical symbol. that is why it is one the keyboard with numbers and symbols.
Maybe it started with Twitter, as someone suggested above. I have never used Twitter. If I want a short expression of a thought, I prefer haiku.
I am not necessarily saying it is rude, but it just looks and sounds juvenile. It is not English. Nor any other language I know of. I like what Norma said about our native language.
If you want to direct a comment to someone, why not just say their name? Or if you want an intro, "hi" or even "hey" would be preferable. ("Dear" is way too formal and sounds almost sarcastic in this context. Or how about "Thanks, x, for your comments".
Anything but that stinkin' @!!!!