If someone saying something that in your opinion isn't true, you should appeal to points that are invalid by providing arguments, not by attacking his playercard.
If you are too lazy to provide arguments better don't post at all.
If you don't ready to listen to criticism - don't post either.
+1000
seriously though... wait i was being serous. i find it is most often used in a fashion that isn't helpful and destructive to the forum.
"hey i don't agree/like(/losing this argument) with this guy, lets check his playercard" -random forum user
the problem i have is its used as a way to often falsely prevent discussion instead of just being apart of the discussion. if someone makes a claim that is false that directly relates to his player card. eg: how many games hes had with x faction) The player card can be used as evidence but it certainty isn't an end all trump card that is being used as here far to often. (there are plenty of things the player-card doesn't track. such as multiple accounts, non auto-match, beta time, etc...
tl;dr: If you aren't willing to have a discussion. forums may not be the best choice for you.