Let's take an actual example: shadowpriest + rogue vs ret + dk.
Let's say you do sap ret, oneshot the pet and proceed with ks > dot > mindblast swd on the dk. Pala gets out of sap, repetances your rogue, dk pulls sp - priest disperses - they pound the rogue. Hard. Let's say rogue vanishes and gets some breathing time - they turn to the priest, priest tries to run (fade), gets gripped then gnawed (by the new pet) while ret and dk are pounding the sp. All these "dk is out of runes" is so good in theory but fails utterly in practice. Let's not forget that DK here has AMZ, AMS, Icebound + Bone Armor, Lichborne, Death strike (5% healing per disease, 3 diseases) and can eat the pet and heal up for 50%. Now, there's no way you can:
1) Nuke it
2) Run away from it
3) CC the paladin (or dk)
4) Nuke the paladin
You can theorycraft all you want, the most skilled priest and rogue will lose to mediocre dk / ret because the arena mechanics aren't equal for every class. Also, disc priests are so oom after only a few moments of the fight and have the most retarded mana regen mechanic. Innervate > all.