But it's not just your chance to hit, if you sacrifice 1 attack to get advantage on the second you don't get to add any bonus damage. An ogre does 2d8+4 damage, even if you increase the odds of hitting (more on that in a sec), you are now only going to do 1 potential hit for 2d8+4, not 2 potential hits for a total of 4d8+8.
As far as sacrificing 1 attack to get advantage on 1 attack, that will never be a good idea. Yes, you increase your chance to hit once, but you also have to calculate in the odds that both dice will give you a result that would have hit. If I need a 15 or better to hit, with advantage I could easily roll 2 dice with a 15 or higher each.
Right, same reason
true strike is the worst spell in the game. And since this is a soft rule, when the PCs are fighting one lone ogre, it doesn’t have to blindly follow this rule. However, if the ogre has allies that don’t hit as hard, it’s often a very good move for one of them to use its action to grant the ogre advantage.
Now, depending on the exact circumstances - how much the ogre and its ally both hit for, what their attack bonuses are, what the PCs’ ACs are, etc. this might be a net gain or a net loss in expected damage output. But the point of this rule (more of a guideline, really) isn’t to optimize damage output, it’s to put the DM in the mindset that, given the choice between attacking without advantage or using its action to create an advantage for itself and its allies, a monster should generally favor the latter.
Sacrificing 1 attack to get advantage on 2 or more attacks may make sense depending on the odds of getting advantage. Trying to knock someone down isn't guaranteed.
Sure, it’s a risk. But it’s a risk that makes the combat more dynamic, so it’s probably a good one for the monsters to take when they have the opportunity. This shouldn’t override common sense tactics, but it’s a good rule of thumb to help prevent combats from turning into slog fests where everyone stands still and throws d20s at each other until one side runs out of imaginary “don’t get killed” points.
Weaker enemies using the help action may make sense and is something I do. That ogre? There's a goblin running around distracting the PC so the ogre gets advantage on their next attack is something that happens now and then.
Yeah… I feel like the help action is a little against the spirit of this rule. Like, sure, it works to grant advantage to a heavier-hitting ally, but I think the point is to think more about how the monsters can use improvised actions to gain the upper hand. At least describe something interesting the monster is doing to help its ally, if nothing else.