In D&D 3.5 there is a Massive Damage rule:
If you ever sustain a single attack deals 50 points of damage or more and it doesn’t kill you outright, you must make a DC 15 Fortitude save. If this saving throw fails, you die regardless of your current hit points. If you take 50 points of damage or more from multiple attacks, no one of which dealt 50 or more points of damage itself, the massive damage rule does not apply.
Preface
I do not like this rule, I think it is a bit silly. But that's just me. In my campaign I have modified that ruling to still have some consideration when Massive Damage is dealt.
I have come up with the following alternative:
If an attack deals at least 50% of the remaining hit-points of the target, the target needs to make an appropriate roll to see if it doesn't get "taken down".
Here meaning in most cases a Balance roll, but could also be a Ride roll, Climb roll, etc - as circumstance dictates. This basically means that Massive Damage in my games means there is a chance to knock down an opponent (which I also consider as eligibility for Cleave)
My question is not about this alternate rule, but knowing the context is of my question is important.
As my campaign has progressed (from level 1, now at level 12) the players and their characters are much more powerful, and are able to deal 50+ damage on most attacks, this has caused some of them to wonder if we should use the proper rules for Massive Damage, stating that it is part of the rules, and the design of the game.
I understand that argument that certain spells, or attacks are absolutely more effective if there is a chance that they can kill targets with Massive Damage, and while my alternate rule gives a benefit, nothing beats an instakill.
Question
So I want to know, how integral is this ruling to the game? Is playing without this rule acceptable? I never considered it to be an important part of the game, but is it possible that I am mistaken, and that much of the games design and balance is expecting this rule?