This is a useless metric IMO because there's so much subjectivity in getting merits, and the fact that you can receive up to 50 per member per post.  Yes, in some cases it's useful in demonstrating who's a good writer (like nullius), but there are just too many nonsense variables to even pay attention to.  I think probably a lot of people feel that way, since I've seen this mentioned before and no one seems to think merit:activity is significant.