Tanks tend to be concerned that they "feel squishy," and taking less damage and needing less healing will make them less squishy. While you might not be able to track precisely how much of a damage reduction you had, or how much less healing you needed, you can track uptime of the buff. I agree that making it clearer would be good (I recall some similar tracking discussions about resto shaman mastery when it was introduced).
This section here is something that's central to the concerns I expressed about AM earlier - that content is being balanced around a certain uptime and without that uptime you'll be "be squishy." That isn't necessarily a problem in theory but I think it definitely is going to be a problem in practice, and the reason why is because the tank AMs operate differently and will likely continue to operate differently, for 'flavor' reasons if no others, since that apparently is sufficient justification in Blizzard's eyes not to provide all tanks with defensive cooldown parity either.
The upshot is that different tanks' AMs will be better or worse for different content at the current rate. At the high end we might see this playing out like the differences between tanks in DS, where within the same raid, some fights favor some tanks over others - which is a problem in inter-tank balance, but a 10 or 25- man group of dedicated players will likely be able to find workarounds and strategies (or just do chinese fire drills with their specs). And frankly it's a problem that Blizzard has shown, via recent history, that they're simply willing to tolerate existing and leave it to the playerbase to deal with.
What concerns me about the tuning problem actually is that in 5-mans, if the content isn't balanced at a point that's so low that any AM usage is practically optional, we might see certain tanks lagging far enough behind in 5-mans that they can't readily handle the fights. There's definitely a disparity in effectiveness on the beta between shield block and shield barrier for warriors, and between SotR and WoG for paladins, so that they both remain much stronger against blockable physical attacks than unblockable and magical ones, for example.
Edit - and tanks who have to build to 3 HoPo or 60 rage before using their AMs will be starting most encounters behind the 8-ball, without access to their AM at one of the most dangerous parts of a pull, especially a trash pull - the beginning of it. When all the trash is alive and all their abilities are off cooldown and ready. I'm sure the pro top notch people don't care about 5-man trash, but for a lot of people, that's going to be most of their gameplay, and it's going to be everyone's 'introduction to endgame tanking' at 90 - doing their normals and heroics for gear. If they're feeling too squishy against Magic Damage Encounter X, or worse, getting votekicked by teams for being too weak, I don't know if they'd continue trying to tank for long. I do know they'd probably not be having a lot of fun.
Death knights will be pretty good against everything still, though, thanks to the many things Death Strike does. Maybe the solution here is to make the other tanks' AM buttons do more than one thing, or at very least, make their physical and magical AMs equally effective.