It gives a sense of pleasure and happiness on getting merits and for the whole day i remain full of joy.
Hm. That's probably an extremely unintended (but positive) consequence of the merit system, something Theymos probably didn't anticipate--that receiving merits would make someone's day. And if getting merits can make your day, that's probably true for many other members as well.
And yeah, I like receiving merits because it's validation that I'm making posts that either people agree with or enjoy reading. I'm long past the point where I get excited about it, though, and in fact I just wrote in another thread that I'm encouraging members
not to give me merits, but to save them for lower-ranked members who make good posts and need them to rank up. I can't rank up any more than I already have, so merits don't serve any functional purpose for me, nor do I really need that validation anymore.
Even as a Sr Member, every Merit still count. Maybe it's just some mere number but hey,
e-penor 
Of course; even for Hero members merits still count (assuming they have less than 1000) since they need them to get to Legendary status, the holy grail of ranks for members who registered shortly before or any time after the merit system was rolled out.