If your worry is that creating new accounts will help trolls, the obvious solution is to create lower limits that are in the positives, so that new accounts won't be able to post either. Nothing onerous - say, +10 lifetime for Discussion, +100 for Main. Certainly removing the barrier to -karma posting is a bad idea.
It came to my attention that when you receive downvotes for your comments, your karma goes negative and you need to "pay back" to be able to post to Discussion or to Main.
Since new users start with zero karma, having negative karma seems to just encourage those with negative karma to create a new account. We don't want to encourage people to create superfluous accounts, do we? Therefore I think LessWrong codebase should be patched so that karma does not go below zero even with lots of downvotes.
What do you think?