I prefer the pet do about 1/3 of my dps and offer different forms of utility based upon which one I was using.
I'll admit I'm still salty about DH taking our our meta/dark apotheosis and getting imps on felstakers and kamikaze imps to replace it. The only thing that would replace demon form is like, summoning 30 succubi or something. Even then... :\
Warlock should be stronger than their minions, if they are summoning a minion stronger than themselves then you get the ability "Enslave Demon" where we force it to obey us but cannot permanently make it obey us.
Considering how much we have changed since Vanilla though we should have access to significantly stronger demons. In vanilla we had the ability to use felguards. Now we grew enough to learn how to permanently control an infernal and doomguard but if we look at our story from vanilla until now then we should be on the level of Pitlords and Nathrezim, even if it is temporarily binding them to us like we use to do with doomguards and infernals.