There are different cases for this. IMO, people shouldn't:
a) Outright beg for merit.
b) Outright beg for post reviews via PM.
I do not think that we should instantly tag people who fall under the category 'b' (which is not the case with OP). However, if the user displays a pattern of asking several members for this, we should tag them IMO. What do you think?
This is fair, but obviously it's gotta be looked at as individual cases.
For instance if someone that is actively advertising a service where they review posts, and they receive a PM to tell them to review/merit the post in question, then I doubt that there is much damage done.
But if there is a guy who goes around and PM's everyone that has replied to his topic for merits, then it qualifies as unsolicited spam IMO. And that, obviously, should be a negable offense at least in my opinion.
Begging for merits is essentially the same as begging for trust. Not much difference as both are indicators of an account's trustworthiness and quality.