It seems like a good idea, but it doesn't work. It just leads to karma-farming bots and shit like this:
Probably such limits help a little bit, but they're like the "bathroom door lock," they only stop the really casual offender. The organized disinformation farms certainly aren't hindered by it.
What I'd like to see is some kind of more informative reputation system than karma, like the "web of trust" concepts that have been floating around for a long time. I'd like to be able to see that a bunch of people whom I marginally trust not to be bots have labeled some account in some way.