That would design WHM into a destructive loop if healing resources are locked behind hardcasting & spamming a DPS GCD mechanic, mainly for situations where you do need to heal because of a bad run / people taking extra damage & dying everywhere, but your heals would be locked behind a lot of DPS GCDs in situations where you don't have time to DPS.
Quickest example I can think of is if you died and lost all your gauge, unlike a RDM where they can just build their DPS meter back up and just have a DPS penalty, dying as a WHM would mean you lost a huge portion of your healing kit & healing output that you may could planned on healing a different mechanic at that time, which defaults you back into using Medica II / Cure II with a huge -25% stat debuff and mana cost. This implies means you won't be spending time to DPS since you don't have resources to get everyone's HP up as fast as your healing resources are still locked away via not DPSing, which ends up being in a destructive loop of no GCD DPS and more Medica II spam (and more MP expenditure) to catch up. This is in addition to heavy movement phases where you are forced to move and can't hardcast for a while - which postpones getting your lily gauge up.
On another note, it also means that while the Afflatus healing spells could be MP-less to cast, it's definitely not free since it cost you a couple of DPS GCDs to unlock the spell - and DPS GCDs do have an MP cost. Afflatus spells now incurs a hidden MP cost which is deducted from your natural MP regeneration. This MP cost is negligible when the battle is fine and you're supposed to DPS to help out, but it becomes a burden when you aren't afforded the luxury to cast your combo in succession as it means less MP-less heals in the long run.
tl;dr : This design seems good for complexity but it's also very punishing to a WHM if you die once because it puts them even further behind when they're trying to catch up.