Literally the only role that is 100% mandatory. The only time that a ranged adc is not 100% necessary is when you have 
, who alternatively has Puff the Magic Dragon and his bonkers ass Q. Seriously, how do you look at things like say, Tank top laners, and think adcs have the lowest impact, 1k damage autos apparently have no impact. Ya ok.
You. Can't use Mordekaiser as an example. Once this shit hits live. Morde Bot will completely disappear. His top might as well cause graves.
I hope he gets down to a .01 % pick rate so Riot Can see they Completely fucked him
Correction: "ADC's have the least impact when shut down, or played incorrectly."
Most people think an ADC is as risky as an assassin that has high damage and are not incredibly, such as Master Yi, Katarina, etc. But they overlook a few key facts, such as the fact that an adc is ALWAYS duo laned with a role that is almost specifically designed to 1) secure kills for them, and 2) make sure they don't die. They also overlook the fact that an adc's range makes them a lot safer than an assassin, because they can deal tons of damage from a distance, so it's not like an assassin that has to get in somebody's face, burst them down, and probably escape from the minion wave/tower/enemy team before their squishy ass gets wrecked.
Overall, anyone who says that this role is not important or has no impact is either lying to themself because they're bad at adc, or just doesn't know how to play this game.
ADC's do have an impact but they require a lot and when played incorrectly is horrible. They need a good sup who will keep them alive or setup kills, a tanky top laner to take a lot of damage, a mid who ganks or keeps the enemy mid from ganking and a jungler who needs to gank. Plus they need minion kills so yeah they don't have a huge effect some of the games I play only last 20-30 min and to do 1000 damage takes along time they carry the team but generaling if the game ends earlier then 45-10 min its your team mates doing so good work pushing and feeding. Yes its like 80% mandatory but i have had games with no adc and they made me go adc zac which I went apc and we won. A ADC is weak and needs help early game and do a ton of damage later since you can be shut do fast that's why you need tanks or your graves who goes tanky.
Rangers are required because they can:
Number two can be covered by having some combination of two AD elsewhere (Mid/Top/Jungler). If Nashor's OnHit worked against towers, Mages would be able to fill in for number 1 pretty effectively. Number 3 is fiddly without some kind of EXP cheating mechanic.
ADCs aren't required because they're better than everyone else. ADCs are required because of how Summoner's Rift is designed. Over in Dominion land where Towers don't have to be auto-attack ADCs are just as optional as every other class.
