I believe that the term 'useful' is relative and changes from person to person. If something is useful for me then it doesn't mean that it should be useful to you too. I agree that it will take time to inculcate the habit of giving merits but that doesn't mean everyone should contribute considering that thing in mind. I believe that if my contribution is helping even a single person then it is a good day for me and gives me a feeling of satisfaction. Everyone has his own views regarding merits but one should never stop sharing his knowledge in the best possible way.
Yes, you're right. the term " useful" depend on the definition of each person, you may find it is useless but for other, it has the usage. Who knows? I believe that they have their own reason when they do that.