Well only one of your upvotes on one post would be worth about 0.01mrt, so you'd have to upvote one user 100 times in order to give him one merit, therefore much more people would have to agree with the post being good or helpful or whatever, therefore that post would get at least merit partially since there's a chance like this that it wouldn't get merited. We all know what's the definition of merit, and a lot of people finding a post useful or helpful or informative is enough to be at least acknowledged.
Sockpuppets would upvote each other's posts and honest users would be at a disadvantage. At the very least this would incentivize absolute garbage posts because in order to e.g. rank up to Jr. Member one would need to make 100 posts and upvote them with an alt, or 50 posts and upvote with two alts, or 1000 posts and upvote only every 10th post to evade detection, etc.
I would rather see some sort of "send for review to a merit source" button but even that would need protection against abuse/spam.