totally agree with this point - I nearly sent a merit to a bounty spammer earlier because he made a funny post.
So why didn't you merit it? ...merit is to be given for high quality posts and not
post history.
In an ideal world I would agree with you that each post should be considered on its own as to whether it is worthy of meriting, but in this particular world a single merit is all that stands between those 1000 post newbies and the ability to spam for sig campaigns once again. Hence why one should have a care before smacking that merit button.