But more than double the merc’s dmg output and tankiness
More useful than some small quality of life auras
Most builds don't care that much about their mercs dps compared to their own benefits from aura, especially since its gimped by 75% against bosses
The benefits are also a lot more marginal than 'doubling'. On a basic might merc you get ~530% ed baseline, so getting ~150% ed on coh (since most are undead or demons) or 300% on fort alone, would be like about 24% and 48% more overall damage respectively. If he's attacking a 50% dr demon, a protectors stone can wind up being 25% more damage just from its +20 avg / -10% dr. There's a lot of PvM where a greyform with protector's stone in it does about the same DPS as a Chains of Honor, except for the higher leech on CoH
dps on fort/andys isn't anywhere close to being worth the benefit of cure