To me, anything above 7000 Armor and 700 AR will have significant diminishing returns, where % reduction from gear will make much more sense.

However, if the player can afford the gear, the more the merrier of course.

I thought the fact that either RA or Armor had diminishing returns was a myth. People just looked at the flat reduction % and saw that it shrank and assumed it was diminishing. But when actually doing the math, in fact it was linear. I thought what WAS true was that you get to a point in Resist All or Vitality that stacking one gets you more EHP since the other is so high (not including Armor since that one is limited by what can roll.)

I could certainly be wrong here, but I thought if you had say, 1000 Resist All on a WD, and got to 1300 Resist All with Gruesome Feast, that that 300 Resist All got you just as much damage reduction as the rest did, but in gearing it might make more sense to stack some Vitality or Armor if you are typically getting 300 Resist All from Gruesome Feast (say a 0Dog WD who has full GF stack most of the time) simply for the previous comment about them adding more to EHP at that level. But not because it actually reduced damage less effectively.

I dunno I could certainly be wrong, but I ran a few numbers through and it seemed to indicate this (both on d3up, and just doing some damage calculations.) Not that I came up with that myself, I thought I read that somewhere early on.

Edit:

This old post was one I think I read:

http://us.battle.net/d3/en/forum/topic/5149150485