On sites where I either have 10k rep or am a moderator, I'll sometimes see a clearly-bad post -- incoherent, not even remotely an answer, etc -- that's already been deleted. So yay for the immediate problem being fixed, but not all deleted posts get downvoted first, and sometimes deleted posts get undeleted. The OP, in particular, can undelete on his own if he's the one who deleted, and sometimes -- shocking, I know -- members of communities disagree about what's "clearly bad" and might bring back something I would have downvoted. Further, undeletion doesn't bump a post, so that clearly-bad post might sneak back to haunt the site later. What I'd really like to be able to do is to add my downvote1 when I see the post, even though the post is already deleted, just in case.
As a side benefit, if that post does come back later, I'll be alerted to that fact by my -1 rep (if it's an answer), so I can take a look to see if any action is called for (remove the DV because it was fixed, comment, flag, vote to delete, etc).
As a moderator I could, in especially-dire cases, undelete the post, downvote, and then delete it again, but that feels like an abuse of the mod powers, plus it clobbers the deletion attribution, plus this doesn't help 10kers at all.
As with any other deleted post, reputation changes from the vote, for both the recipient and voter, take effect only if the post is undeleted. This is not a proposal to do additional reputation damage to somebody whose post is already deleted.
1 For fairness's sake, it should also be possible to upvote a deleted post. This could help in the case (which I've seen) where somebody self-deletes a post he thinks isn't good enough and others try to persuade him to undelete -- some votes waiting to be claimed could be additional encouragement.