Yeah in line with how caster jobs tend to work, this will simply be the spell you cast now instead of curagaIV. Just like you use curV, curVI, instead of cureIV for various reasons, in the event of needing to cure 4-6 pt members of a massive aoe quickly (incase a 2nd massive aoe follows) whms will now cast curagaV instead of curagaIV. After conserveMP, gear, sj abilities etc. the extra 120mp should not be an issue. Especially because, like its been said, the use of this spell will be generally rare.