I cast Unholy Frenzy on my Warrior who has Rusty Armor and Mail. He blocked the damage using the Rusty Armor, which meant he lost potential damage saving for later. If he had used Mail (same roll requirement, but KEEP keyword) instead, it would have been far better. I assume armor use is determined in order of newest to oldest, since the Mail was the oldest of the two in his hand. It would be nice if it had preference toward Keep cards.
If they change it, then I'll be sure to use Shredding Strike against you. But yeah, as has been discussed in general, there are MANY possible combinations of "I'd rather this card triggered before this one" in the game. Keeping it to a "left-to-right" rule will be better in some situations than in others. But this way it is more predictable and less complicated to decipher.
Hrm... that one does make it so sometimes you would prefer to reserve the keep I suppose. And of course further complications if you have a couple of weak armors (like reliable hides) and then a strong one, and everything gets revealed to chip away at damage which winds up being over-defended in the end, but leaves you with lots of visible cards, thus vulnerable to other attacks.... Ok, so if the rule sucks/rocks no matter how you do it, I suppose the easiest to process and remember (current) is sufficient.