If you don’t know what spell batching is, it’s the way the game processed spells before WoD. Spells were processed every server tick instead of ASAP and it’s what let two Mages polymorph each other, two Rogues gouge each other, two Priests fear each other, and so on.
While it’s true that it was originally design because of technical limitations, it was kept all the way until WoD because of the positive effect it had on game play, most notably PvP.
In PvP it gave you options that are not available to you now under current game mechanics. Gouging your opponent’s kidney shot, frost nova and jumping your opponent’s stun, vanishing a death coil, and any other interaction between spells that would be exclusively possible on the old system and not the new could be used as example.
In vanilla it was what prevented Gouge+restealth from working. With a 5.5 second incapacitate and a 5 second combat timer, in theory it should have been possible to restealth before the target recovered. But in practice someone mashing an instant cast skill could “tag” the Rogue before/while he restealthed.
The list could go on, but without spell batching, the PvP won’t be remotely authentic, the meta will change and class balance will be altered.