Thorn was garbage, and now it's an Anti-Fighter item. It does not function against marksmen because the people who typically build this item are easily kiteable, and they typically have a crit steroid.
Start buying bramble vest against Attack speed / AD / [insert health regen mechanic here]. The magic damage / attack speed slow is meant to counter the attack speed these champions get, and the wounds counters the lifesteal.
Think about it like this. Tanks are like engage supports. They Engage -> CC -> Try to zone / peel in fights. They are countered bys sustain damage (Design that gets rid of counters is bad.) This item really helps them against bruisers and fighters that are ahead, and is an alternative for when they don't want to itemize against the ADC.
If you're interested, you don't buy Thorn against Marksmen is because you're losing the cold steel passive from randuins. It's inefficient to stack the passives, especially when you're an early / mid game class that makes things happen before people naturally scale. Though, when they do scale with their AD / AS / Crit stats, you have Randuins and you get your HP / Armor faster. (Randuins gives HP / Armor / Crit reduction and Attack Speed reduction in the same item! That's an INSANE early game stat stick that scales!)
Marksmen really need 3 items to be effective... Tanks are great from levels and 1-2 items. Personally, i think randuins is fine at its job.
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is not very good
has 83% gold efficiency and other item that are ENTIRELY used for this passive like 
also have 80% efficiency
