@joefromchicago,
Actually, tagging it the same thing does add to the folksonomy, and suggesting previous tags is actually an idea we plan to implement (when there is more tag history).
Like this: at least one person on this site is already deliberately tagging things with nonsensical tags. One of these topics about a2k was tagged "politics" and another "poker" and so on.
That's where the correct use of others is expected to outweigh it. So for example, if the vandal tags incorrectly but then 3 people tag it well, the vandals tags may be pushed off of the public view (the "topic tags" are the top 5 tags for the topic for e.g.) or outweighed by the better tags.
So if someone goes and tags one of your topics "totally ******* awesome" it would show up prominently till enough people tagged it differently. So even if someone already tagged something, tagging it again helps keep it tagged that.
You are right in that unimaginative people (or simply people who just wanna bookmark something quick and haven't put thought into meta data for the topic) might not come up with great tags, and to make it easy for them to use tags as bookmarks we'll soon make easy ways to add "suggested" tags that are based off of the existing folksonomy (easy) or off of semantic analysis of the text (hard, but something I want to do because I like this area of algorithm).