You're a tank and you're fighting using FD or CE fighting a baddy and it's going a little badly. You can instantly switch to Parry mode to up your defenses, turning FD and CE off. Great! If you've been wise with your skill points you now have +10 ac and the healer in the party gets some time to help you out. Now you want to switch back to actually pummelling instead of hoping for a riposte (maybe you wear full plate and don't riposte that often) but now you have to deactivate parry before you can activate FD and/or CE. Meaning you're vulnerable for one or two hits between canceling Parry and activating FD or CE.
TLDR; Please make FD and CE cancel parry instead of us just getting server messages that we cannot use them with parry. It will save us some time in a real time game. It's not like activating FD or CE while parry is active is likely to happen on accident. Just assume we know what we're doing, ok?

ETA: Is the cooldown on FD really necessary? I don't understand the purpose of that.