Unnecessary bumping is frowned upon because bumping sends the last post on page 1 to page 2 (which is tantamount to the kiss of death on Daniweb). But you're correct that there's no rule against it. Legitimate bumping that continues a discussion is fine, though it still annoys certain people. ;)
Special circumstances require special handling of course, and if someone regularly bumps with pointless posts that are clearly for the sole purpose of sending their thread to the top of the list, I'd say that it's fine to close the thread and explain our take on bumping.
In the case of the post that's linked to, I think it's fine to bump a 2 day old thread that hasn't really seen any action. I wouldn't really call that abuse. I've seen some desperate homework-seekers who bump threads every hour with rude comments.
It depends on the context. If it's a reasonable continuation of the thread, all is well. However, if it's just a rehash of the original question or only tangentially related then it should have been posted as a new thread and falls under Keep It Organized.
Keep It Organized if it's a complete thread hijack. However, if it makes sense as part of the thread, then it's fine. I would rather have my question answered late than never. It can still help other people even if the OP doesn't need help anymore.
To conclude I think conclusion is that if somebody is doing baby cries for help every couple of hours or demandanding response now or immediately it is not Ok, otherwise OK if it does not happen too often to same poster.
I would however do progress update of my own progress toward solution and that would bump the thread more effectively and give better impression for helpers. Also it shows that I am working on it and the thread is not abandoned. Which should motivate better help from others.
Also, some people don't like if someone resurrects a really old thread (over a year old or a few years old) with a response, even if the response is valid and quality. Personally, I think this is perfectly acceptable and I almost encourage it. Like I said, better late than never.
Urrgh. I think we'll have to agree to disagree on the necroposters. Nasty horrible egomaniacs as they are.
Necroposting is absolutely fine if the intent is directly related to the thread/post/question concerned and is a logical entry route into becoming an active member. Necro-spammers and post-count-inflating-tossers are another matter :)