So do I. I tend to upvote the sensible (i.e. not crazy) ones, in the hopes that someday they'll make it back to positive karma.
The rationale is that trolls and spammers will usually register new accounts upon being notified of a ban, but a hellbanned troll/spammer might continue to use the hellbanned account without realizing that their posts are ignored.
* The test for a comment being auto-killed (marked [dead]) is "(or (ignored user) (< (karma user) comment-threshold*))"
* comment-threshold* = -20
* Karma is still updated on item authors if their post is dead - as long as the tests on the person doing the voting pass.
* Once you have the ignore flag set, only administrators can remove it - it is independent of karma.
* You get the ignore flag set for posting a link to a banned site, posting a comment containing banned text, or if one of your posts is blasted or nuked by an administrator.