I don't see any value in Guardian Angel on AD Carries.
If you die, it's because you're in a bad position. If you're in a bad position when you die, you'll probably still be in a bad position when when you're Revived by its Passive.
I'd much rather get Randuin's Omen or Banshee's Veil if I'm getting dived because then I actually have decent effective HP against Magic/Physical Damage.
With Runes, Randuin's Omen and innate armor I had 190 Armor and roughly 2.7k HP + BT Shield + Sona Shield on Caitlyn against an AD-Heavy team and ended up 12/4/11 as Caitlyn. I mean, they had a 350cs Nasus would could barely harm me and their Varus was getting 3-shot by me while he was taking 10+ to even get me to half HP.
, who I build Wit's End.
It prevents them from diving you, killing you quickly, getting their reset, and messing up the rest of your team.