Is there any reason to build armor?

Yeah... if you ever run into a fed Rammus, Mundo or Malphite, you will see the reason to build armor. Maybe it's less effective than it was in the past, but it still works.... OK.

Last Whisper and Black Cleaver are both % armor. And not everyone is going to be building Thornmail and Frozen Heart. There is barely any option for armor beside big tank items.

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%maxHP true damage is there to counter armor users. Kinda stupid to comment on armor being useless against it. Yes its worth buying armor as only one of those items penetrates % of bonus armor. Rest are just flat armor penetrations which are not as good against Frozen Heart or Thornmail. As for %hp true damage its conditional for Fiora and Vayne. Vayne every third hit and Fiora by hitting vitals. But u still reduce their regular attacks. And Vayne isnt strong cause of %HP true damage but because of his AD steroid on ultimate.

To be very honest here, the way to counter armor users should be magic damage, not some free damage steroid that literally ignores it. Something that would reduce the armor potential, as long as it doesn't kill the purpose of buying armor is fine.