Before the release of Bungie.next, there was much discussion surrounding the implementation of a like feature. People presented the many pros and cons of such a feature. However, there were always multiple employees who seemed to support the idea. I think that this was more or less a compromise. I could be wrong though. It seems more sensible to just have it for the OP to help with thread sorting. As people said in the past polls really limited activity in a thread. If every post was likable, it may give members an overwhelming reason to post less, therefore limiting overall activity.