Assuming "average" rolls of 50 over the course of your whole career:
Keep in mind that "A hit" is roughly at +25 net OB. . .real nastiness starts at +50 net and you max out the table at +100 OB net.
So at -50 to parry, what was a perfectly fair fight last round at net 0 OB, has become a beating at +50 OB net.
Just throwing out 100 Skill as the level at which these two fighters were fighting, no parry is akin to the next round going to +100 OB net and the top of the table.
So there's really no need to pile atop that.
At very low levels, where OB's scale below 50, you do "lose" some of the penalty as you can't parry a negative amount to make yourself easier to hit. . .if your OB/DB split is 25/25, being -50 takes you to 0/0, so your DB has only fallen 25 not 50. . .so there's some justification for saying that the penalty is nerfed a bit for low level combatants, but that's probably OK, since RM is deadly enough for greenies that it's probably OK that the net-net tends to be more like 25 or 30 rather than the full 50 or 100 potential net loss higher level combatants face.