This talent is particularly useful when combined with Fel Domination which allows your next Imp, Voidwalker, Succubus or Felhunter summon's casting time reduced by 5 seconds and its Mana cost reduced by 50%.
One of the best uses of this is when your Voidwalker is near death, use its Sacrifice ability to give yourself up to 30 seconds of protection. Then cast Fel Domination and summon a new Voidwalker or a minion that is more appropriate to the situation.
Another use of this talent is if you're a Dark Pact/Demonology talent build using your Imp as a mana battery. Whenever you summon a pet it comes back with full mana. Even without 5 ranks in Fel Intellect, your Imp's mana pool is larger than what it takes to summon him normally. With the 40% mana discount provided by this talent you can suck your Imp's mana dry, re-summon, and drain it dry again for a net gain.