I'm guessing it is due to the fix to a previous issue that caused PCs and NPCs to cure themselves of all effects when succeeding a save throw upon application of a new effect, i.e.: a bleeding and burning character would cleanse himself of all these effects if he succeeded a save throw against stun. I could be wrong though.
I still find the save method odd... doing a bleed attack for 2 turns, and it does no damage since it's shrugged off right away? (most times with my wolf) Of course, enemies saving against bleeding will skip their turn afterwards, but that's just a bug on top of a bug (on top of a bug?) I also expect if you can freely replace effects it makes it much easier to stunlock an opponent indefinitely, and that's already fairly easy to do, especially freezing (with the only downside being increased armor of the opponent).
