So here we go...
Asheara's Vestments, four pieces, summons all missing follower's to your aid. This means, in a multiplayer game, it summons all three. In a single player game, if you've got one out, it summons the two missing ones. The followers are your actual followers; they've got the gear you've given them, and they use the abilities you've chosen them to use.
The followers stay out for 30 seconds. The summon procs almost instantly at first, but there's some internal cooldown, so they won't appear for another half a minute or so once they're gone.
Now... Use Unity yourself. Give Unity to all three of your followers. Unity splits all damage taken between everyone who wears it. Have the "cannot die" follower special on each three of your followers. Apparently, if I understood it correctly, a follower who "cannot die", also cannot take damage. This is why people are using the Unity + immunity legendary combination at the moment, to gain basically a permanent 50% reduction in all damage.
However, with Asheara, and three followers out, you would basically have a 75% reduction in damage every time the followers are out. Which is pretty often. Furthermore... With Asheara, you can utilize Unity in a group setting nicely too. Normally you won't have followers in a multiplayer, but with Asheara, you will.
Thoughts?