There should be a public list of all merits given/received by a user, like the trust page.
I was thinking the same thing, but as Darkstar said if they delete the post/thread then what record would there be to look at other than just the +X amount? If it links a deleted post, it just won't be there to look at and observe. Unless there is a safe way to utilize archives without squandering forum security.
Yes, not seeing the target post is problematic, but if someone has a lot of merits for deleted posts - that would be suspicious in itself. Like spammy trust ratings with no reference.