If you want to limit it to thread creator, I don't see that helping since they will treat it just like a title, and will probably be to lazy, or create unnecessary ones.
If you want to open it up to admins and mods that could work, but at the same time its not like they don't have enough going on as it is. Having to clean up tags would be tedious, having to be the only ones tagging the threads would be unrealistic.
That leaves letting anyone tag the thread, and I think I just showed why thats a bad idea. Actually TGR got rid of their thread tagging for the same reason, It just stopped being helpful. The reason TGR forums have a better search function is because the code is written way better then the one NS uses.
If you want the search function to work your just going to have to bite the bullet and write all that nasty code.
Or you could get over seeing reposts.