What is the issue?
Between mobs there is usually enough time to regain mana via natural MP regen, however you throwing up MB shouldn't really be an issue, please remember you are a support class. What you are saying is no different to me deciding "hey that dps could have dodge that hit or popped some CDs so I might not decide to heal him because hey maybe they just see me as a unlimited supply of health.
If the healer is keeping people alive and doing DPS at the same time, be happy they aren't just sitting there but are trying to speed up the run.

The reason for using holy on mobs regardless on number is because it does the most dps, stuns which in turn reduces the mob damage output.

Personally I dont ask for MB, I dont expect it so I manage my mana so I will always have enough.