Don't have a problem with this as long as you really are consistent and down vote Clippy, Quirrell, ect.

Yes.

Would be interested in a link to your rationale, which I feel you must have written out somewhere.

Yes, somewhere. Two of the purposes of the karma system are:

• To discourage people from posting undesired comments. You outright declare that this is what you are trying to subvert: "In any case its kind of the point of using Charlie Sheen to say stuff that will get me down voted but I think need to be said."
• To give an indication of whether the posts of a user are likely to be worth reading, as well as how good the user is at thinking (useful when choosing how much credibility to assign them.) This is also subverted both in terms of karma score and direct associations with the user.

The even more obvious reason is that users who are logging in with multiple accounts will end up voting multiple times on the same comment. This is obviously not good.