I agree that this rule is not in your favor.

Szega's answer covers the probability distribution pretty well, and I believe his math is sound. Markovchain's math seems sound, and he generally agrees with Szega, but I think his conclusions are slightly off. I still generally agree with him. To summarize the points they agree on, when your GM rolls a group save, the expected value of the damage you do does not change, but an extreme outcome is guaranteed.

I believe that this rule is not in your favor because extreme bad outcomes are much worse than the median outcome, but extreme good outcomes are only slightly better than median. When you get very unlucky, you die. When you get very lucky, you use fewer resources.

Take an example where you throw a fireball at a group of orcs. With your GM's rules, the spell does either half or full damage. Your party either still has a tough fight, or they mop up the remainder. For a single fireball this is not a huge deal.

Lets extend this example. You are in the final fight against an especially tough group of orcs. You throw a series of fireballs. With the standard rules, there is a very high probability that the damage your fireballs do is somewhat close to the expected value. With your GM's rules, the orcs will probably fail some and succeed some, but the chances of succeeding or failing ALL their saves is much higher than standard. In a boss fight, you can get TPKed if they save all their rolls. If they fail all their saves, the fight ends slighly sooner.

You have more to lose from an extreme bad outcome than you have to gain from an extreme good one.

I agree that this rule is not in your favor.

Szega's answer covers the probability distribution pretty well, and I believe his math is sound. Markovchain's math seems sound, and he generally agrees with Szega, but I think his conclusions are slightly off. I still generally agree with him. To summarize the points they agree on, when your GM rolls a group save, the expected value of the damage you do does not change, but an extreme outcome is guaranteed.

I believe that this rule is not in your favor because extreme bad outcomes are much worse than the median outcome, but extreme good outcomes are only slightly better than median. When you get very unlucky, you die. When you get very lucky, you use fewer resources.

Take an example where you throw a fireball at a group of orcs. With your GM's rules, the spell does either half or full damage. Your party either still has a tough fight, or they mop up the remainder. For a single fireball this is not a huge deal.

Lets extend this example. You are in the final fight against an especially tough group of orcs. You throw a series of fireballs. With the standard rules, there is a very high probability that the damage your fireballs do is somewhat close to the expected value. With your GM's rules, the orcs will probably fail some and succeed some, but the chances of succeeding or failing ALL their saves is much higher than standard. In a boss fight, you can get TPKed if they save all their rolls. If they fail all their saves, the fight ends slighly sooner.

You have more to lose from an extreme bad outcome than you have to gain from an extreme good one.