Ahm, btw, a think that should be an advantage fro the posting system:
Member-evolution: If some member was 1337 at the beginning, maybe s/he's gonna change with the time (what happens quite often, people get used to the forums and so on).
If you already rated this member (member-system), it's very unlikely that you re-rate him/her unless s/he did a big change. In the vote-per-post system, as you keep rating posts the tendence of the rating is constantly changing. So, if the member is evolving (positively), in a way you just don't notice, your ratings for him/her are getting better and better (in
my system, the average would grow slightly ending up with a real rating that shows my overall impression about that member even after his/her evolution).
Hope you got me (my English is getting worse since I'm here...
)