Really? I can not imagine not having them at this point, the better the system gets the more I use it... It's let me merge many forums into one at this point by using prefixes to describe the thread content... For example I used to have a General News, World News, U.S. News, etc etc forum all seperated because people would just jumble it all together, but then when prefixes became a little more user friendly I merged everything into a single news forum and required a prefix.
I also use them for staff members to make posts Admins and Mods can use the Staff Notice Prefix so people know that it's probably important to look at.