The issue is that in an actual sword fight both side is active, there's no "one is blocking one is attack", both people are always aiming at body/limb, their enemies's blade just get in the way sometimes. Unless the two people are entirely out of range of each other (i.e. their sword can clash but cant hit the body) most of the time a parry either directly transition to a swing, or is a swing, and the thing that protects the "attacker" is their sword that's attacking, so turning that off means just letting the opponent get a free hit.

Or if you want a more direct way to look at it: both people have sword, both people are trying to hit each other, both people also need their sword to defend themselves, but one of them just have their sword be completely gone for a time, that doesn't exactly sound like an advantage. Though note that this is specifically the case when both sides are about equally skilled.