I'm ambivalent about this, as there are definetely downsides (on other forums, you see people posting contoversial opinions or pointless attempts at humour just to get "likes").
However, to me the most useful aspect of having a "helpful" (far better term for FT than "like" IMO) button is so that I can later track the posts that I have found helpful. Sometimes you see a very useful piece of advice, which you don't need now but may need later. By flagging it as "helpful", you can then easily search it later from your own profile. This exists on moneysavingexpert.com, which appears to use the same technology as FT. There are a lot of pointless "thanks" on there though.
For me it would be enough if I was the only one who could see which posts I had found helpful, I don't need to see what others have liked, nor is it important to me to see my own posts liked (though I hope they are of some use to others).