It does what you want to do, but it's too tailored for specific situations like (4 ADs in their team). It faces stiff competition against Randuins, which, not only protects against AD but also AP thanks to the large amount of HP it gives. Similarly, it pales in utility compared to Frozen Heart.
Thornmail is a very selfish item, but it does work.
When life gives you lemons, blame yourself for planting the wrong tree.
It's fine where it is. It's a situational item, but there are situations for it. Certain characters just need it more than others.
Singed for example has reasons to build it over Frozen Heart and Randuins. It's more for characters that you KNOW the enemy ADs will be forced to focus. Singed will run up to the AD carry ignoring everything else, so the AD essentially HAS to shoot him.
GFAQs sheep are gonna throw Randuin's and FH at me, but challenger tier Singeds (such as InvertedComposer and IAmLegendSmurf) avoid FH because Singed just does not need the utility it provides as opposed to the ability to murder a carry. Randuin's is a bit more debatable, but Singed doesn't need THAT much armor, and Thornmail is more important.
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I get it all the time. It's cheap and provides alot of armor. People just refuse to get it because they either compare it to way more expensive items or they always plan for full build (which you rarely get to do).
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