I'm with Ana & Absent I think it would be the sort of thing that just ended in "point scoring" rather than people actually saying WHY the agreed or disagreed with a post.
OK, the way things are at present, does mean that sometimes there is quite a bit of wandering from the OP, but surely that is better than just metaphorically ticking a box?
I do understand that there may be members who disagree with my point of view, but I sure as hell would rather that they articulated their opinions than tick a sodding box, or use a symbol!
(Tin hat on, bunker ready warmed, try doing that with a symbol!)