People respond to incentives. Especially loss-related incentives. I do not give homeless people nickels even though I can afford to give a nearly arbitrary number of homeless people nickels. The set of people with karma less than five will be outright unable to reply - the set of people with karma greater than five will just be disincentivized, and that's still something.
The prior probability of someone being able to explain negative-value Will_Newsome's comments in a way that provides value for LW readers is already epsilon. Even without the disincentives.
I think that people less responding to intentionally meaningless comments is a good thing. Therefore, trivial disincentives for doing this are a good thing. Therefore, removing them in this specific situation is a bad thing.
Here's the new thread for posting quotes, with the usual rules: