Ah, but FF13 didn't have MP. It didn't matter that spells didn't have tiers.
Bringing that up because it'll come up, you can bet on that.
The biggest arguments I've seen involve "I have no/little MP but I want to do damage" and "I want to do damage and not get too much enmity".
Enmity, we have no idea how these new abilities are going to affect that. If spells generate the same amount regardless of damage dealt, then there's really no issue. If it does then there'll be a scale for that, that is hopefully easily adjustable in the event that "it doesn't work quite as intended". Plus, you never know. These new class traits might just help towards that kind of thing anyway. CNJ/WHM may get 'Tranquil Heart' like traits to cut down enmity gain on healing. Either way, being mindful of your enmity and not continuously throwing out low tier cures.
This system also lends itself to more strategic game play. If (and I'm hoping this is so) PLD gains abilities to look after themselves and their party members (own healing abilities and Cover etc) means you can let your PLD get his target's attention and you go from there. It also makes using other tanks more and more risky. Sure, your MRD/WAR can tank but he will need help from his healers to do so, thus having an impact on enmity control and strategy. Being able to just hand out cures without thinking much doesn't sound fun. Maybe this system will promote thinking first.
As for not having enough MP, that comes down to play style. I prefer to not have my MP run low, and if it does I focus on trying to recover it or use it more efficiently. Being an XI career RDM taught be to savour each MP I had. If I'm in a healing role, I'm not going to be doing any nuking. The party depends on me having MP to heal them, so that's what I use my MP for. It's called team dynamics. And the same principle applies in XIV. Either use MP more effectively, or carry a lot of ethers around if you insist on extra duty activities.
Also, as I said before, MP used (and in turn, enmity gained) should reflect the damage done/healed. That'll improve this system a lot.