This varies from community to community, I reckon most communities don't mind people bumping old discussion topics if they have something new to add to the discussion, which has the potential to revive the discussion thread again. Otherwise the alternative is usually a duplicate topic.
On the other hand bumping a thread just for for the sake of pushing it back to the top of the thread list with no content like "bump" is usually frowned upon..
It depends. Over the years, people tend to change their mind about things, have new experiences, things that were said in the past might no longer be true in the present, etc. Hence why some feel it's better to create a new topic over bumping an old one. Other than that it's just preference, I guess.
Information in an old thread may be outdated, therefore, bumping the thread can cause confusion. If the board is a support forum then it is definitely a bad idea because the issues may look the same and turn out to be different versions, different problems, etc.
No, because if the reply was just a "I agree" then it would re-open it for another x number of days. For most "problem reports", new versions resolve those issues or as said above, the data is outdated due to newer versions.