For me, there are actually quality posters who aren't legendary members but may not be so consistent and as such it could make you a bit confused.
Legendary tend to speak/ post based on their experience and knowledge - that's more meaningful than the average user who is trying to build an account and earn merit from here and there. So it's true that more experienced user who rank highly have more meaningful quality posts than others - but I'm also not generalizing them all to be the same.
The posting quality standards for each merit source are very different. They can sometimes consider posts like mine to be quality because they contain elements of argument, facts, or so on. Some of these others are ordinary posts that are of no use being given merit even if your rank is Legendary.
I remember going through a thread where someone said the gambling section needs merit sources too because inasmuch as there are spammers there, good posters can also be found but the thing is most of these sources may not have the time to start searching.
There are several merit source on the gambling board - but more often than not they distribute the merit on specific thread. The main problem is probably the quality standard of the posts - I mean, they really struggle to find good posters in the many thread that already have hundreds to thousand of page.