You seem to have forgotten Regen effects (HoT) exist.

Let's say a raidwide knocks the group to 10%. The next raidwide is 30 sec away. In this example the WHM might place Asylum and the Sage would Physis II, then 23 seconds later the WHM would have Assize naturally come off cooldown and heal the rest. The party is full before the next raidwide, no GCD heals were used, good planning and heal coordination.

With your change Asylum is almost useless and only there to buff Cure III, which the WHM frantically spams to get the party back full to avoid losing raid buff uptime, and then the later Assize and any HoT goes into massive overheal. Not a well planned or coordinated play, but had to be done to keep the damage buff active. Doesn't sound engaging.

I can see the thought behind the idea (and respect a good discussion), but our toolkits are not designed for always keeping parties topped off. Not only HoT's but look at abilities like Essential Dignity or Excog. Or the fact that if you GCD heal someone to full right away, then an oGCD heal comes off cooldown before they're in danger, it was a waste of a GCD. Topping the party immediately just isn't a good play in this game.