The punishment would be a lot less severe if

a) Fester and Bane could only be used on targets with Bio and Miasma, and
b) If we didn't lose all the Bahamut stacks on death.

Being able to target Devotion would be very nice too. We have a series of role skills that are used by targeting other party members, so why can't Devotion do the same? If it's to intruduce an element of randomness to the all-purpose buff, then the right macros can nullify it almost entirely... so why?