For a lot of tanking situations, stamina is more versatile than avoidance since it's works on all fights, especially fights with heavy magic damage. With the
[Chill Of The Throne] debuff in Icecrown Citadel, the dodge on the Hodir enchant is effectively decreased. The side benefit of the PvP enchant is that the resilience helps you reach uncrittable, so you need less defense.
There's a pretty involved
post over on the Maintankadin forums where they determine when to swap out certain gems and enchants from avoidance to stamina.
The shoulder enchant swap is pretty low on the list, so it's not something you should be doing as soon as you hit 80 or anything.