It really depends on how much sMerits you have, I usually merit when I learn something from the post but I don't have enough meri9to share whenever I wanted but users who are merit source or have huge chunk of sMerits to spend may have different policies but its okay to give as much as you can unless you're abusing it, and don't forget on few cases merits reversed by theymos if I remember it correctly.
When I was somehow new, I used to have sMerits and I used to merit any post that I learn something. But as time goes I understood that I was learning many things I do not have sMerits to reward everything that I learnt. So, I started to remove my mind from giving merits.
If there are merit sources who are distributing merits, atleast it should reach every good poster once in a while.
The merit system is fine, it helps to motivate people to maintain healthy conversations and also
prevent reduce spamming.