Pants are pretty lackluster for SMN. "Pet:" stats are all beaten by other gear. Whatever edge Cure Pot. had is lost with the outrageous HP in Abyssea these days, a couple dozen extra points won't make or break any efficiency threshold, let alone the actual fight. Which is why I'm tossing my hat+pants and remaking them.
Best bet for SMN is an idle piece, Haste, PDT, and Move are all decent combos (Move not so much now
). I'm looking at Haste+PDT or Haste+Acc, but that's for my RDM really. Hat gets better SMN options like Perg said, though I still might make a WS piece for RDM or MNK out of it, it just depends on what the game does with the next couple updates.
Avatars have a native 50% damage reduction already, which would make another 10% all the more noticeable.
If I remember right (reading people messing with 1000 Needles), gear is applied to damage after natural damage reduction, meaning you're knocking 10% off of the remaining 50% that gets through. So "Pet: Damage taken -10%" really just acts like "Pet: Damage taken -5%". Natural reduction makes it less noticable, but still might be worth it if you tank a lot.
As an aside, you can get Haste+5% from Spurrer Beret, only major reason to get it for Selenian is if you want the Haste+DT combo specifically.