Well, critical success gives you some bonus above and beyond a normal success, so what else is a critical parry going to do for you beyond blocking your opponent's strike?
GM: The force fields deactivate for two rounds.
Qui-Gon: Attack! I use the spin-thrust manoeuvre! 14!
Obi-Wan: No! Wait! Listen to him!
Qui-Gon: Reverse parry! Critical!
GM: A critical parry, huh?
Qui-Gon: Yeah, he hits himself!
GM: Let me just look that up...
Obi-Wan: Do I make it past the force fields?
GM: It's eighteen hexes. See if you're fast enough.
Obi-Wan: Ah. I don't think I make it.
GM: The last force field cuts you off from the fight.
Qui-Gon: You and me, bounty hunter! Bring it on!
[SFX]: Whoom! Whoom!
GM: He rolls a critical pirouette.
Qui-Gon: Oooh! I'm learning that when I level up!