Full Disclosure: I lost about 300 points when the correction happened a couple weeks ago. I then just this morning saw that I gained all/most of it back. I know several times I deleted my posts when (a.) I later found them to be partially incorrect or otherwise inferior, but (b) they had garnered a lot of votes. I'd be worried the OP would go with my answer, and I'm a little embarrassed to be the top-voted answer when it's clearly (a little later) not the best. If I later found my answer to be inferior but had only 0 or 1 upvotes, why bother deleting? It was already getting the votes it deserved! Not sure if this accounted for all my points: Maybe 1 or 2 of my other answers were considered dups and deleted by a moderator.
I certainly agree with Gilles about not deleting "outdated" material. That should be kept indefinitely if not forever. If a would-be archivist wants to improve the tags to make clear the previous version or tooling for which it's relevant, then they are doing a great service. I also agree that the 3-or-more cutoff and been-around-a-while is not a "balance" that helps.
As for duplicates, I would suggest the poster be given half-value (5 points per upvote) for any gained rep, no matter how much. This is a balance that I believe would motivate people to take the most-desired behavior. It certainly would with me. I believe myself to be a typical somewhat-selfish, yet somewhat community-oriented and helpful stack-overflow user.
I deleted my own posts at first because I found myself to have really misunderstood the question, yet had gotten votes. (The Disciplined badge helps that first time of course.) Like making your first kill, it was easier later, but always somewhat painful because I knew the question would, if left, get me more rep. Once I saw that it didn't seem deleting my posts cost me the rep already gained, it was certainly easier to do! It cost me a little, but not a lot, and it was the right thing to do, so I did it. If I lost all my rep for deleting my own post, hmmmm, well, I would do it if the situation was really bad, I guess, maybe, but I'd be a lot slower to do it. If I was going to keep half however, I'd certainly want to be a good citizen.
And if I got half-points for an answer to a duplicate question, that would eventually be deleted, I'd (a.) not be really pissed off if it disappeared later. And you know, I did really help that guy, and I should get some credit for that. But if I only got half-points, I might be more motivated to not jump in with an answer, but do the research to close it as a duplicate. That's the main thing we're after, right? We want to decrease motivation to answer duplicates, to answer off-topic questions, but still give some credit for helping people quickly and effectively, and yet not completely punishing people who mistakenly answered duplicates.
Imagine you're a new user, answering questions you can, all excited, and then you get caught in a crackdown on duplicates and/or off topics, and you see your points drop from a hard-earned 180 to 70. You are pissed off and de-motivated and not having fun. Well, if it instead dropped down to 125, your not as pissed off at least, and that's good. Bottom Line: If someone quickly and effectively helps the OP, and is recognized for doing so, completely removing their rep later is a bad move. Reducing it, on technicalities, is not nearly as bad, is fair.
It's balance. Is losing 1 rep for downvote fair? No, but it's good balance. Is 10-for-upvote but 2-for-downvote fair? No, but it's good balance. Partial credit for deleted content is good balance.