Why would the devs create such an item? It brings no value to any magic using classes. At 95% magic resistances we are not going to be able to kill anything efficiently. At -50 magic resistance, we cant counter this amount.
Furthermore there’s no way to reduce this number in the current state beyond a marginal amount.
The only magic immunes that do magic damage are the greater mummies… As you said, the 5% Damage done to them is nothing so it’s useless.
The melee one is also garbage. It provides a 5% Damage increase over just having amp/decrep procs at a massive cost. Physical resistance is the easiest to break…
You forget Blessed Hammer damages undead 150%, so its far from useless.
Which is still irrelevant… Merc will be the one to kill the mummies…
You would be better served wearing Gore riders or goblin toes and smiting them 
The Bone Break (physical) is not irrelevant. They nerfed decrep now with bone break repeapers toll works the way it used to. Pointless nerf to occupy a charm slot though