I'm not trying to complain but just curious. It feels like the lifesteal from Bloodthirster completely negates anything you do even with 
+ 
+
+ 
. Had a game where that was my build, and no one who had blood thirster ever took damage. Is it even worth it to do Thornmaille? To be fair, the person that I'm talking about is a Jhin... so like... Everything free there. I guess I'm not sure if his missing %hp damage is factored into Thornmaille
There's a chart somewhere that shows when armor is effective vs buying health. Basically once you have so much armor, it becomes more efficient to buy health. As far compared to life steal Jhin is usually very squishy so if you catch them, it should take a while for them to kill you so you can kill them. If they're really out healing your damage with all your sustain, consider buying executioners calling. For only 800 gold, you cut their health in half.
It's good on rammus because it works well with his kit, but generally it's not very good unless the enemy team has several auto attack reliant champions. Also the biggest problem with the build you put is that you have almost no health, the more armor you get the less effective it is so you would have had much more survivability building an HP item instead of thornmail.
