WHM No Cast Time Heals

[RyuujinZERO]

I'm not seeing OP's concern.

If the tank is in serious danger of dying before you can stop and cast: Benediction
If tank needs to be kept propped up but you can't afford to stand still for a while yet: Regen
If you need to cast a spell urgently, but situation isn't dire enough to justify benediction: Swiftcast

There's already a pretty robust set of tools to allow for healing on the move. Plus in 8 man scenarios (ie. HM trials where movement is the biggest thing), you'll usually be partnered with a scholar, who also bring heal-and-scoot skills to the table (Lustrate, proxy heals via fairy). Anything more and we're moving away from the caster vs martial division found in games like this and need to remodel game mechanics in terms of GW2's twitch based game