D&D 5E Variant rule oversight?

[Dausuul]

This would mean a lower bonus or penalty at single (2D20's), double (3D20's), and triple advantage (4D20's)...about equal at quadruple advantage (5d20's)...and then a higher bonus or penalty than rolling extra 6D20's or more.

Not true, at least for attacks and saves. A flat bonus can never take you above a 95% chance of success, because a natural 1 always fails. With five levels of advantage (6d20), if you have at least a 40% chance of success on the base roll, you're doing better than any flat bonus no matter how high. Also, how exactly do you plan to stack up five levels of advantage? There are a lot of sources of advantage in the game, but not so many that you can easily bring five of them to bear on a single roll.

Anyway, I'd guess the reason they didn't include it as a variant was that they didn't think it was a good idea. Blackwarder's experience sounds about right. While stacking advantage does obviously incur diminishing returns, the first 2-3 levels provide quite substantial benefits. With a base hit rate in the 60-70% range, which is typical, three levels of advantage is essentially a guaranteed hit. If you're using a limited resource such as an assassin's surprise-round autocrit, you'd certainly pull out all the stops trying to reach that guarantee.

If you want to do it, you can. Nothing's stopping you. The DMG is not the be-all, end-all of variant rules. It's just a list of variant rules that Wizards has vetted and approved. They don't seem to approve of this one.

 

[El Mahdi]

deleted

 

[Riley37]

There are times when an incentive to stack and keep stacking makes the game slower AND less fun.

There are also times when denying incentive also makes the game less fun. For example, say the party caster uses Faerie Fire on all the foes. Good tactics. But now no one has any incentive to use their action to Help someone else's attack ... even if it would be appropriate, under the circumstances, for "create a good story" purposes. And that might be time for the DM to consider allowing the +2, or perhaps just a +1, to the Helped PC's attack roll.

When you're rolling two attack d20s, +2 on *both* of them seems like a LOT, but I haven't playtested this.

 

[aramis erak]

Riley: a DM does something VERY different than advantage does. Advantage reshapes the curve; a DM doesn't, but shifts the curve and the range up.

	Adv	Std	Dis
Mean	13.52	10.5	7.47
Median	14	10.5	7
Mode	20	—	1
StDev	4.92	5.92	4.92
Min	1	1	1
Max	20	20	20

Advantage or disadvantage changes the way the curve is shaped, but doesn't change the range (minimum and maximum) at all.

A DM changes the mean, median, and mode, plus the minimum and maximum all up.

 

[Henrix]

At first I was planning to do the same as Warmaster says above, not letting one disadvantage cancel all advantages, and vice versa (but not counting how many and still just rolling 2d20).

Once we started playing I discarded the idea as being unnecessarily complicated. I find it seldom matters.
Occasionally as the GM I decide that some things outweigh the others. Shooting blindly in the dark at a surprised enemy still gives disadvantage, for instance.