What.
Darkrai is amazing. 90 hits all the perfect numbers, Eelektriks, Tornadus + Thundurus with Darkness Claw, 2HKO all EXs. Babies and Tynamos have nowhere to hide from the snipe, and it also sets up other pokemon for OHKOs.
Darkrai.dek and Dark Toolbox/Zoroark Rush are completely different. I don't even think that Darkrai would be good in Dark Toolbox, it does less damage than Zoroark for more energy. The reason it's amazing in Darkrai.dek is because it rushes Darkrai to get quick KOs with the main attack as well as the snipe from Dark Patch.
Honestly, the free retreat is just a bonus. I don't see it in any deck used just for the free retreat. No deck would have the room for Dark energy.
Also, Tornadus EX covers the Terrakion problem.