I don't think a "Like" or "helpful" button is necessary, or that either will provide any substantive and positive impact on FT. On boards where the function is provided, I ignore it. I don't need to know that 7 other people have found a post to useful to consider it useful. I reach that conclusion on my own.

That said, I wouldn't object to seeing a "helpful" button added. If it helps some people evaluate the utility of a given post, I see no problem in adding that functionality.

Adding a "like" button is a juvenile idea, IMHO. It has a sort of Sally Field aspect to it - "you like me, you really like me."