All I'm seeing is my ADC splitbuilding from the start with 
& 
, kept leaving me in wonders what build in the world does he wants to finish first. But before I get to my answer, it was already too late as I see an enemy Twitch with just 
and took down both bot turrets.
it really depends. if his first back he got pickaxe, and then he died/stuggling in lane or was the whole time, and goes back with 1000, he should get something or he will be down a complete item and if the other adc knows what they are doing, they will punish them hard enough that he won't be able to cs or he will die or loss tower or a mix of the three.
just for an example:
your Lucian is vs cait.
Lucian is struggling and dies to cait. cait's first item is and your Lucian has . some farming happens and then some trading and Lucian loses, but live and goes back and gets (either nothing or ) and cait goes back and gets . now, is good for the lifesteal, though he still won't be able to fight cait. the lifesteal would help him survive being poked at much easier. if he gets nothing, then he is a behind cait and she will have complete control over lane and Lucian won't even be able to trade her since she will get her health up faster than him from the lifesteal and be dealing much more damage.
another case is if he had enough for during one of the backs, then he should pick it up. if he's having a really hard time living, then he could go 
. if he's wants more damage, then 
would be better. after getting , he should focus on one of the two. if he goes back and gets both and , it's not terrible, but it's not as good as focusing on a whole item.
If your ADC is unfed, a proper splitpush will at least create pressure to deal with him and at best will improve his CS. This is particularly true if the other team is on the other side of the map and there's not much your ADC can do in a teamfight.
