That's a pity, was thinking about this the other day also. Mainly because of when I add movies and using multiple Prefix for different "Genre". IE: A movie can be Prefixed with "Thriller, Horror, Mystery". But did think that would probably be better as Post/Thread tags feature anyhow, not so much Multiple Thread Prefix. But still, it could work just as well.
So, has Kier also ruled out the option of adding Thread/Post tags in the future? Or are we just talking about Multiple Prefix here only? I saw good results in Google before using WordPress and multiple Tags this way. I could create one article, yet get about 4 pages indexed by Google from it. One, the article itself, 3 others being TAG pages. 4 listings for the price of one entry.