Comments like yes, no, agree, disagree, good post, and lame are concise, but also boring wastes of time which add little to a discussion. The aggregate measure of those sentiments (as expressed in a point total) is both more concise and of some value, as it serves to encourage and highlight good posts and discourage thoughtless or pointless comments. It also provides a relatively unobtrusive outlet for those who wish to voice appreciation of thoughtfulness; this is preferable to the sea of "agree"s and "disagree"s that might spring up in the absence of a point system.