"It's just too juvenile." - not for the BBC it isn't !
Would you claim the same of various newspaper web sites, and other media sites?
I have no interest whatsoever in FaceBook or Twitter but there are instances where a post needs no more than a "yes, I agree" yet if you respond to that effect on a fast flowing thread, it looks daft and can be several pages away.
You hit the nail on the head when you wrote "and you're quick enough" - I expect a number of people would hit a "like" or "dislike" (or thumbs up/down) marker even a couple of days later, but would not fit into the "quick enough" bracket for adding a simple smiley or frown (and unless you are specific about which message you meant, could look inappropriate if something obnoxious had just been posted before you hit 'Post Message' yourself!)
If it's good enough for the BBC, it's surely worth considering, and a fair number of others on MN have expressed an interest, not once, but several times (I've seen it three or four times in the last year or so).
Unfortunately, however many times it has been discussed, it seems to be being ignored in the most part by MNHQ... At least this time the thread did get a little input from MNHQ, but almost THREE MONTHS have passed since there was any comment from that quarter !