Well I am one person who ilke to get it right so I went home and tested Might of The underworld. With 1 pet out Might of the underworld actually increases your total damage output

I couldn't believe it but its true. I was actually doning more combined damage with a single pet when Might of the underworld was on.
You are right. The amount of str taken away by might of the underworld is actually less on the pet than what you receive simply because at evo 1 and evo 2 the pet stats are lowre than yours. So even with two pet out the over damage will not be much different HOWEVER there is one downside to the equation. Might of the underworld will drop your pets def BUT it depends on what type o pets you are using and which aura you are using how detrimental this would be if at all.
For instance I love Blood Aura. Might of the underworld would therefore make my pets very weak defensively. In PvE however my kill speed is so high a mob will get 1 or two hits in maximum before I kill it. In dugeon I seldom get agro so that lower defense is of no consequence + healers are around in case of the unfortunate event that I do get my pets damaged. In PvP however Using blood and might would be suicide so Halisha and blood would be my choices. In fact because I go magic in PvP Might is of no use to me anyway since I use my pets to stall delay people from getting to me but at the same time I need my pets to have maximum damage and defense since they will be getting up close and personal.
So my take on Might of the Underworld now? Use it if you need to! As for me, I actually use it now when I solo.