I'm not disputing what the PHB says or that WotC did itthat way because they thought they would make us a favor.
Ahkay. Carry on
Not only does is it completely gamist, and a complete disconnect between out-of-game rules and in-game "reality"
Don't follow that - 'And I surround you in a nimbus of darkness that deals 5 necrotic per round' 'I'm already nimbused from when you did that last round' 'Oh, right, carry on'
Examples:
1) If you're both hit by a Freeze Bolt that Immobilizes (save ends) and Grave Bolt that immobilizes (save ends), one will completely override the other, despite them being completely different powers; one Cold and one Necrotic. According to the rules, you simply save against the most recent one.
These are different powers so you could easily argue that p278 indicates you save against both.
However, this example is illustrative: if, say, you've got a +5 save bonus against necrotic effects and the Freeze Bolt has a -2 penalty on saves. Then you want to be hit with Grave Bolt last, because it gives you a relative bonus of +7 for your save.
Doesn't matter what order you get hit by them...
In fact, with a scenario such as this, if you have an ally that can produce a necrotic Immobilizes (save ends) effect, you might even want to ask him to blast you in the face as this will automagically "erase" that difficult Freeze Bolt for you, replacing it with the much easier version of the same condition!
And the rules definitely don't say that.
In all cases but the last one, you'd keep taking the ongoing damage even after successfully saving against one effect. In the last one, however, because you've got an exact match on the game statistics (regardless of in-game causality) saving against one also removes the other. And yes, how is this good for the game?
How is it bad? The in-game causality of 'Two mages came up and fire shrouded me and set me on fire, then I rolled and the fire went out...' isn't less sensical than when one did it.
It's a balance mechanism on ongoing damage. Personally, I think the easiest or best method would have been to make it by power name, so two mages Fire Shroud = 1 save, 1 mage Fire Shroud and 1 warlock Flames of Phlegethos = 2 save. I could be wrong, though, because then people would have to write down the names. So maybe the easiest method _is_ in fact what we already have.
1) is in no way related to what's actually happening in the game?
Except... that it is completely related to what's actually happening in game? Unless you want to argue 'Okay, well you're nimbused a second time' 'Oh, so I take 10 damage now?' 'No, still 5. But if you save against the first nimbus it goes away and the second starts to work' is somehow _less_ gamist and more sensical in game.
2) doesn't reflect "common sense" and thus is hard to remember?
Common sense is rarely common, or sense, when applied to a greater population.
3) generally confuses the hell out of people?
They definitely could have written the entire rules section more clearly, with clearcut examples. That's not necessarily a problem with the rule inherently so much as explanation of that rule.
What is the actual benefit of this rule?
Balances attack stacking, encourages targeting multiple people with effects, and less careful individual tracking required. Big benefits.
Sure, if you face lots of identical monsters, you're somewhat protected against focus fire (but not if they simply try to blast you to smithereens) but is this really worth it?
Totally.
