But if you have an MP dump how can the healer be expected to be able to heal or revive when called on?
If healers can turn MP into DPS it shifts the focus of the class a bit too much.
The optimal way to play healers would be for the rest of the team to play near perfectly so you could dump all but the bare minimum MP into DPS... and that would just lead to a lot of friction between players.

I'm also worried about how this would inform how much HP devs would give bosses.
If the healers need to heal a bit more in a run then sorry you don't meet your DPS check. Not to mention this would require a fair bit of coordination between healers to make sure they're not both healing when only one of them can cover something.

Suddenly the game is much tougher towards any mistake and it demands a much higher level of party coordination.
These aren't inherently bad things, but I don't think this is the way to do it.