One obvious problem with that system; what happens with habitually bad posters?
Let's say I write something so insipid and worthless that it's worth every downvote on the site... and then a better-quality poster writes an excellent point-by-point take-down of it and gets tons of upvotes for it. Should I then benefit from "generating" such a high quality rebuttal, or is that just going to weaken the already weak incentive structure the karma system is supposed to be creating?
I can think of a good case just in the last few days of a poor-quality poster who would seriously benefit from this system, and as a long time poster here you can probably think of more.
I don't think that an excellent point-by-point take-down in a comment is not a good idea because
a) It is not very visible and if it is excellent and makes a point it should be done as an independent posting.
b) Writing a point-by-point take-down may be overkill and alienate the initial poster. Compensating him with karma may make good for this (and motivate the commenter to post separately).
c) I think individual counters should be addressed by individual comments to allow the to be voted and commented individually.
In the remaining cases and if the point-by...
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.