Thats the thing, players themselves do the balancing here. With such small percentage boosts there isnt much 343i would have to do. Being tailored to your own play style you more or less just enhance what your already doing, and with the ability for anybody to get any armor in the game players are on equal footing still.
Its a give and take situation. If you spec fully into shields you give up damage, speed, recharge, ammo capacity or any number of other things people could think of. Things that can nullify your fully spec'ed shield. Just like choosing loadouts. You have several different ARs that while fill in the same role have different stats to them and affect things differently however slight. Thats players balancing players right there with those selections.