The problem with "Instant OFF GCD" abilities makes combat feel less rewarding.

[RyuujinZERO]

It was my chief criticism in beta, it's slightly amusing that a regen is instant cast yet less time efficient than a Cure III... you can basically only cast a regen on a tank when the tank is at full HP in a hard hitting fight.

Regen is a HoT. Much like a DoT it's all about opportunity cost. While you're spamming cure over and over you're locked into a casting animation and unable to do much else. HoT's and DoT's allow you freedom to take other actions while it's ticking over - if your ward doesn't need a tonne of heavy healing this instant Regen will buy you a grace period before he needs another big heal.

Arcanists are a class entirely built around that principle - DoTing up targets, allowing you to properly analyse the situation and adjust tactics accordingly. So as a white mage you should be familiar with that