#1 - March 9, 2015, 12:31 p.m.
Both Spirit and Bonus Armor are intended to help restrict the number of people in a given raid who may be interested in a particular item. The OP makes a perfect argument as to why we want that distinction: he wanted the Blackiron Micro Crucible, but had resigned himself to only really having a shot at it from a bonus roll or a cache as there were so many others in his raid group who wanted it too. That's a crappy situation.
Bonus Armor is currently working well for creating that distinction. DPS specializations don't get anything from it, but it's far and away the best stat for tanks -- so much so that tanks will nearly always prefer an item that grants Bonus Armor over an item that doesn't, even if the item with no Bonus Armor is much higher in item level.
Prior to 6.1, we were getting a ton of feedback from tanks who were extremely disappointed to receive trinkets, rings, etc. with no Bonus Armor through their bonus rolls. It felt like a waste of a seal or cache to get something that your loot spec would realistically never equip. Plus, it's a bit of a false lead for inexperienced players; if Blizzard gave me a higher item level crit/multistrike amulet via bonus rolls, it must be an upgrade, right?
We got similar feedback from many healers, which caused us to take a look at how Spirit was functioning. Ultimately, we want Spirit to work similarly: a stat that's so good, you want it everywhere you can get it. If that's not the case for you, we'd like to fix that by making Spirit more attractive, not by making you want to equip non-Spirit items (and fight with half your raid over them).
We made some changes along those lines in 6.1 (such as the Mistweaver changes to Mana Tea). We feel that, with those changes, we're now in a position now where healers should always prefer Spirit pieces in the ring, neck, and back slots. The nature of trinkets makes them a little less cut-and-dry: in the vast majority of situations, you should prefer the Spirit trinket, but there could conceivably be a situation in which a DPS trinket would give better performance. We're still discussing what (if any) changes would need to be made to prevent those niche occurrences, but again, our goal is that you don't want the DPS trinkets.