Ecth,
IMO the parry is not a modifier but a usual tactical assignment during combat, which leaves things such as flank bonus, a good tactics roll etc., which are - unless way talk about "monsters" like a house cat - usually far lower than the OB+DB of the monsters.
I'll try to logic this out once more.
1) I think we can agree that negating your DB has the same effect as boosting my OB?
2) Assuming any rational non fanatic puts OB into parry (not always true) If I have OB/DB of 30/30, and you have OB/DB of 40/40, but choose to parry at Paas. . .now, I attack you from the flank. . .your 30 points of parry are negated, since they're not directed at me, plus I get my 15 bonus. . .so 30/30 vs 40/40 (or 60 vs 80) suddenly is 45/30 vs 40/10 (or 75 vs 50).
3) So what before the fight looked like +20 OB/DB in your favor, via a casual, happens every combat maneuvering tactic, has become -25 OB/DB in my favor. . .a swing of 45 points. . .or 150% of my starting OB, or 75% of my starting OB/DB. . . .
4) When your casual variance can fall into scales of 75-150% you're way into fuzzy terrirory. . . .
5) You'd expect this to moderate as you go up levels, but the effects of parry negation cause the variance to keep scaling up as the parry scaling keeps rising.
6) Flank attacking to negate parry is VERY common, in fact, it's the standard tactic of a party attacking a single large nasty monster. . .if the kobolds/goblins/orcs/city guard who are half your level but twice your numbers never return the favor on the PCs, then you're playing them dumb. . .if the monsters are assumed to always be stupid, while the PCs are always assumed to be smart, it eliminates much of the variations that might create problems.
Good luck and I never hated this idea, I just think you're going to have serious problems coming up with anything clean and easy that offers more information than merely comparing monster levels as is. <Shrug> whenever I'm up to something I prefer contrary feedback to people gushing about how wonderful my idea is, as critical comments are often far more constructive to making something work than agreement ever is. . .It's never meant in a hostile manner, but I guess what I prefer in terms of critical idea feedback isn't universal.