To make the doomguard and infernal more useful or likely to be used, they could change them a bit... make them require more mats to summon them and, with the doomguard, nobody dies. However, make so they are "enslaved", so to speak, to begin with. After X amount of time, have them just vanish/unsummon. If they were like this, I'd use them.
Or even better...
With this upcoming expansion, add a demonology skill that allows us to have a permanent control over the doomguard, as if it was just another summon, like felguard, succubus, etc. It could be something like a more advanced enslave demon that lasts forever.