as long as their is account activity in the past x months, then i agree that they're using their account even though they dont post. i think its the combination of no activity and no post that require pruning. also low post count (less then 5?) with no activity is another reason to prune in combination with no activity in x months.
not that dead accounts bother me in anyway, and if anyone cries wolf about a dead account having their name.. well obviously that person has already registered and would switch names, leaving another dead account (is it possible to have multiple accts on the same ip?)