I play a lot of adc, especially Vayne, and I know she is a late game champion and needs a few items. I don't currently use Cull because I don't see other people build it, but I don't understand why. Why don't you buy Cull on scaling champions? Have I been doing something wrong all this time or is it just an underpowered item?
Cull is essentially the "I want to get to mid game safely" item for an adc that can farm. It's safe because using its stats only requires you to last-hit minions and will give you back its cost in 100 cs. This is unlike Doran's blade, which lends itself to trades (via HP and lifesteal) with the enemy duo.
You don't see Vaynes build Cull due to her kit's design towards trades (auto reset, 3 hit passive proc, hard cc, invis on ult). She also has short range which makes farming much harder to do without engaging in a trade so you probably won't end up getting the Cull gold passive as easily as you would with Ezreal or Caitlyn.
I think your mistake with Cull is thinking that it lends itself to scaling adcs. Instead you should ask yourself if:
- a) You don't want to make lots of early trades in lane
- b) If the champion you are on can safely and quickly get the 100 cs required to proc the Cull passive.
If those are both yes you're fine getting a cull. Also Doran's into cull is pretty good if you find yourself behind in lane because at that point a) is true and b) might not be true but farming might be all you can do if you know you will lose trades form being behind.
The reason you don't see it often is because Cull is an item that is somewhat risky. It is very gold efficient, since it literally pays for itself over the course of the laning phase (meaning you got stats "for free"), but the cost is that you are somewhat weaker than an ADC who chooses Doran's Blade.
Comparison to Doran's Blade
Cull gives you 7 attack damage and restores a flat 3 health on hit. Doran's Blade gives 80 health, 8 attack damage, and grants 3% life steal on hit.
Cull's flat health restore is good for early laning phase when you don't do much damage. However, it is outclassed by Doran's 3% lifesteal as soon as you can do more than 100 damage (since 3% of some number greater than 100 will be greater than 3), which usually happens once you get B.F. Sword or other damage items.
The 80 health difference is also very important to note. That 80 health makes you just a smidge tankier, which usually translates to at least one extra auto attack. In a duel, that one extra AA can mean the difference between a successful dive and a free kill.
And of course the one extra damage helps on Doran's a little bit. Not a whole lot. A little.
Cull's main point of attraction is that it gives you its stats "for free", since once you've farmed 100 minions, you will have earned 450 gold from the item. Heck, if you decide to sell it, you actually turn a profit.
Doran's Blade is a better item if you're planning on dueling or skirmishing. Cull is better if you're planning on farming. Most ADCs take Doran's Blade because they know that farming in peace for 10-15 minutes is rare, and they would rather be prepared for ganks or fights. However, if you're playing a very passive laning ADC and actively try to avoid fighting (perhaps on Vayne, Ezreal, or Varus, for example), it might be an okay purchase.