Its good reasoning to not implement a new system like this for rogues, we dont need more item bloat.
Problem is, is that blizzard has been doing this for years: Healing plate
In a less strict sense with only 2 specs that can use it: Caster Mail, Caster Leather, Tanking Shields, Caster Fist Weapons, Tank Swords (before DKs)....
So on and so on.
There is already too much bloat as is, and blizzard / the playerbase are too afraid to homogenize the armors and shields so that all specs use a single type of armor/shield. Much like how blizzard combined spell dmg and healing into a single stat, spell hit and melee hit into a single stat, spell crit and melee crit into a single stat, etc. , fixing our bloated item system would do wonders to fix item loot issues, save on system resources, and player bag space; as well as layout a system where implementing band new character specs and classes would be a ton easier.
I had a sigh of relief when monks were implemented, because now someone else was able to use all that caster leather. Thats not a good thing to be thinking or design when it comes to itemization.
Aside from that, I dont see a problem with the current system, with adding agi shields and letting rogues use them. This doubles as letting shaman use them as well, so you already broke the (Only 1 spec) clause of itemization. It also means itll make giving shaman and rogues....their "tanky" specs back, should blizzard decide to do so with the possible 4th specs for classes other than druids. Its a win / win in either case.