Originally Posted by Niara
Originally Posted by Rack
Healing Word into a full spell is extremely potent in terms of action economy and Healing Word into Cure Wounds significantly increases the pace of healing at lower levels.

In the lower levels, yes... where your spells lots are very tightly limited, and burning two in a single turn is a big hit to your total long rest resource pool, when you might be having to get through two or three encounters in a day. More than acceptable for the cost. In the higher levels, where HW truly shines as the absolutely worst scaling spell in the game, then it's positively a detriment to be using spell slots on it except in emergencies... it doesn't even pass muster as maintenance healing,, past level 10, compared to the by turn damage output that parties will generally be facing. So if players want to, they can certainly weigh that cost themselves, without needing an extra, counter-intuitive rule that stands outside the general design philosophy of the rest of 5e.
Healing Word at higher levels is primarily used as a way to get party members up from dying. It's the ~most effective way to do so, given it has a 30ft range, is a bonus action, and only a 1st level slot.

RAW, casters have to choose between getting someone up and doing minimal damage that turn (via a cantrip), or casting a high level spell. This choice adds to tactical decision making during combat, and it would be significantly less interesting/tense if casters could cast Healing Word and Fireball.

Originally Posted by Niara
You know what is? Fighter-paladins who can, in the right conditions, blast off eight (Eight) spell slots in a single turn, all converting into guaranteed, rarely resisted damage, and all perfectly legally.
The key phrasing here is "Fighter-paladins" and "in the right conditions." You have to multiclass, which slows your progression in each class and in getting ASIs. And you use limited resources (Action Surge?), so you can't just do this all the time.
Being able to cast multiple spells per turn under specific conditions is different than blanket permission to always cast a spell & bonus action spell every turn.