Not sure I agree, for several reasons.
1) There aren't that many heavy damage situations where shadow resist is useful. It could be useful for heroic Zon'ozz and Yor'sahj progression, but those are the only two with significant amounts of resistable shadow damage. Zon'ozz still does
~75% melee damage, but the Psychic Drains are resistable and dangerous. Yor'sahj's
Void Bolt is dangerous, but only on 25H (it's trivial on 10H, and not worth gearing around - we one-tank the fight in our
alt run, with a tank that doesn't even have the BH resist trinket). I know some of the early Yor'sahj progression kills used heavy shadow-resist gearing, but that's significantly less important now with the 5% nerf active.
Note that despite having ~50% shadow damage from Twilight Instability,
none of the damage in the Ultraxion encounter is resistable. There's a decent bit of shadow damage in Blackhorn, but none of it is particularly tank-threatening. It's generally phase 2 that you'd be gearing for, in particular Blackhorn's physical damage. There may be some value in the shadow resist enchant for heroic madness (Tetanus, Degenerative Bite), but I can't say first-hand.
2) Note that having 20 extra shadow resist doesn't guarantee you anything, because
partial resists are binned in 10% increments - you either resist 0%, 10%, 20%, etc. Adding 20 shadow resistance increases your
average resistance amount from 21.22% to 22.90%, around 1.68% (assuming only resistance aura). But it doesn't eliminate the possibility of resisting only 10%, so it doesn't give you an EH-like guaranteed benefit. You'll just see those 10% resists slightly less often (17.75% of the time rather than 21.95% of the time). With the prismatic elixir included, your average resist goes up from 28.25% to 29.64% (1.39% increase), and your chance to resist only 10% drops from 4.38% to 0.90%, almost enough to push you to a new threshold, but not quite. Armor, on the other hand, is guaranteed mitigation for melee attacks.
Should you consider putting Shadow Resist on your cloak if you're progressing on Yor'sahj or Zon'ozz? Sure, there it's an option. For the rest of the bosses though, I'd stick with armor.