Whether "Must Parry" is a form of stun has been an issue in our group too. I remember something about it being stun from old books, but can't point to where I read it. For most fighters, getting a "Must Parry" of one or two rounds is far worse than getting a "stun" for one or two rounds, since the latter can be dealt with using "stunned maneuover" or similar. Also, the "Must Parry" is definitely worse for undeads and others who ignores stuns (such creatures should really have separate crit tables...). Still, "Must Parry" results always appear at lower crit numbers than stuns. In addition, you never get the "Must Parry" and "Stun" result simultanuously, which would really make sense to do if it's not stun, since the stun can be removed, but not the "Must Parry" part.
Anyway, we use it as NOT being a form of stun when it comes to stun removals and undeads (although personally I disagree with this), but we DO stack "must parry" results with stuns rather than deal them simultanously, so if you get two rounds of "must parry", and next round you get two rounds of "stun", you're stunned the next two rounds and then have one round of "must parry" (the first round of "must parry" was when you received the "2 rnds stun" result, obviously).