I would expect a greater risk in the opposite direction, of the quality of discussion being watered down by floods of comments on stupid topics, meaning that people who don't have time to sort through all the clutter may end up giving up participating in most discussions.

Having spent a few years chatting on karma-less, completely unmoderated fora (spam would be deleted, but nothing else), I can say that this does not seem to occur. The pattern seems to be that when someone says something the forum considers stupid, this is remarked upon, and then they either attempt to improve to be more in line with the general opinion, or leave. People are not really gluttons for punishment - if a community does not welcome them, they (usually) will not continue participating in it - and the ratio of new users to old users is typically very low, so norms are maintained in the medium term (barring major news coverage or something).

Although I guess without the deletion policy discussion may drift further away from rationality, so if you think most of that would be boring or mindkilling, it may be of value.