If the button is handy for 2 users from 2,000 - it's not the feature I'm usually supposed to work on.
Even if we count on the users which found a useful reply or advice, it doesn't make anything useful for the forums themselves. Forums are content. This feature doesn't add the content. It only add some subjective rating (yes, it's very subjective because pressing the button doesn't mean somebody really thinks it's helpful, and such "thanks" buttons could easy become a fake). On another hand, if we introduce "positive" ratings, we also need "negative" then.
But the most important moment is - this feature should be opened for everybody. Not just for registered users. If we make it for members only, it is 100x reduced in efficiency then. Imagine that you found the answer or advice on some forums, which became very useful for you, but you do not want to register to just say "thanks"... most probably if this is opened for members only, the person who really "checked" the reply, will go away with no trace.
This is all is a fake feature. It's not even a feature... it's the feature you saw in other software and think it is useful, and the time you don't have enough of facts to prove it. At the time the feature needs a lot of development and engineering.