The problem lies in "what's the rule before I decide?". . .if you're going to rule based on the GM's opinion, you better give a really good pre-game briefing. . . .it doesn't really matter what the rules are, but it needs to be somewhat clear before hand. If the players don't know them in advance, then how can they make choices?
If I know ahead of time that I can't parry that elephant, then I will make sure to have the mage kill it with a lightning bolt from 100' off, if I find that out after I sneak all the way on top of it, then the urge to throttle the GM might be strong.
Getting into relative values between 1:1 and 1:0 that are not firm and up to the GM means you better be ready to answer the question "If I sneak all the way down to the bottom and then end up fighting that big dude with my dagger, are there any rules you're going to pull out of your arse after the fight starts?" and stick to it. . .and when I pull my dagger two years later on a similarly armed dude, you best come to the same off the cuff choice as to what my DB ratio is, or again, we've entered that zone where considering throttling the GM will arise.
RM is a dangerous game, which means you have to play tactically to stay alive, which means you have to have some sort of grasp on where the rules lie before you start making decisions. . .which means when you decide to do something you should at least be in the same chapter as the GM, even if you're not on the same page. . . .which tends to mean that Occam's razor is a good idea. . .i.e. keep the rule something simple the players can keep straight in their heads. . .don't go with "The GM will off the cuff a call on the relative value of if you can or can't parry at what penalty" rather "If the GM decides you can't really parry strength to strength, you parry at 1/2" so at least when you're making the choice to start climbing down there, you know that the worst possible case scenario is "I parry at half" so you honestly can make that choice to start climbing down, and not be forced to accuse the GM of leading you into a trap where you were screwed and stuck once you got there.
Even "You may never parry X on Y combinations" as long as it's clearly explained up front, is a clear and straight rule the players can work from.
If it ain't fun, the players will leave, if you're lucky they won't throttle you, and arbitrary GM rule changing from instance to instance is definitely one of those things that enrages players.