completely valid, but that is common everywhere. Perhaps there is a formula to calculate a quantifiable measurement of forum activity.
percentage of the average number of posts of all posters who posted in a given period.
lets say in the last 30 days you posted 15 messages. and the average number of posts from everyone who posted a message within the last 30 days is 30 then your rating would be 50%
If you couple that with the car rating system you can assign additional weight based on forum activity which will virtually elimate the non-posting voters who are dumping the 1's on everyone.
So at the time of the rating you had an activity rating of 50% and you voted a 7 your vote counts as six 7 votes and someone with no activity votes 1 it would count as 1 couple that with the natural log and you end up with the 1 vote just about being thrown away.