The problem is that deletion isn't just for duplicates, nor does it always happen in proximity with the last time people looked at the post. And when you're under 10k, there's no way for you to know that was the cause. If I lose a bunch of points thanks to spam flags or heavy downvotes, I can check my reputation report and see that there.
But if some votes I received 3 weeks ago were deleted and I suddenly see my reputation drop by 30 points, then I'm absolutely confused because I will have no clue where it is from. When I see an immediate drop, I expect that I can click the envelope at the top and it will tell me "You took -12 reputation because you actively oppose gratitude", and similar items. This is not revealed when a question is deleted, though, because deletion basically rewrites your history and says those votes never existed.
Because I check my reputation report often (I've requested 2 recalcs, once each before 2k and 3k to ensure that my climb was 100% legit, and I currently have 10 extra reputation), I personally may not be confused by this. But there are a lot of users, especially people who answer a question in good faith without knowing it is a duplicate, that will suddenly see a reputation drop and be completely confused about it. You can't even easily explain to these people which post of theirs was deleted.
At least in the current system, a delayed deletion doesn't have an impact. A user can check their report and see "Oh, my reputation is off. Something must've been deleted". But an immediate drop does not imply that. They will check for immediate causes, and upon not finding anything, be reporting here quite afuming.
The biggest problems occur when you haven't been to the site for a while, and your reputation has shifted a lot besides the deletion. A person might see their reputation jumped by 30 points since they were last around, but in checking their envelope, see that they had a post upvoted 6 times and awarded 60 points. 60 != 30 yields fuming.
Automatic recalculation is healful to the system, but until users are able to track these deletions of their posts directly, it will be very harmful to the community. Until that point, I am opposed to this idea.