Yes, clearly spoken by an ADC main here. Obviously you know everything a marksman does. /sarcasm
News flash, the champions that you play (
) are burst champions. You can normally murder something in one spell rotation. Where as it takes an ADC maybe 10 autos early game to kill a squishy, even more if there are shield or healing you have to counter. So, let's just assume the ADC can attack every half second, so it takes them 5 seconds to kill someone. I'm pretty sure hitting QWER is much shorter than 5 seconds. And most champions in the game can do this to a squishy, which makes having to outplay for a minimum of 5 seconds very difficult when 3 different enemies are throwing abilities at you.
If there is a big wave threatening our tier 2 top tower, I'm going to react to it so we don't lose the tower. If you decide we need to do dragon right then, then that means we may lose this top tower if I come down to help you. Or there is the wave top threatening a tower and my top laner is invading the enemy jungle trying to contest their buff. If I run over to try and help save them from the 1v4 they are now in, we may be able to get a few kills, trade kills, or feed the enemy while all the while losing our top tower. Or I can get this wave and hope my teammate survives their dumb invade.
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I will admit, I've had a jungler gank and the ADC just stands there farming while I run up to help the jungler, but those have been few and far between. Most ADCs want to help but can't simply because they don't do enough damage and get flamed for it, they step forward to do minor damage and get caught and get flamed for it, or they farm and hope to win that one team fight later once they do actual damage.
I've also had a jungler who was full build specifically take farm from a 5 item ADC/support, so don't act just like ADCs farm stupidly when they are full build.