With a voting system, idiots who don't have any good reason to support something have equal say as everyone else, because one of them posts something stupid and they all vote it up.
Without a voting system people actually have to defend what they post and be reasonable to have any say.
For example: If you had a poll in a topic (where no one could reply) about the user Kaje and the options were along the lines of "Cool guy" and "Loser" you might get a lot of "Loser" because the majority of people might be uninformed and not know who he is. However, if you had a discussion about the user Kaje, you would get a positive response, because some users know who he is, and those who don't would have no backing to call him a loser and would quickly join the informed as the discussion progressed.