Spellcast users use max -BP precast and the alternative armors midcast, so it frees up nothing to people with third party tools.
I was under the impression (and the wiki does nothing to help dispel this) that BPs operate like all other pet commands - they're JAs, not spells as such, and therefore there's no "midcast" to them.
Of course there's a midcast to Bloodpacts. How else have Summoners been able to macro in -BP first, and then +Smn. Skill between activation and animation?
They are job abilities, but not instantaneous like Provoke or Sneak Attack are.
And I'm not sure what Mellowy is commenting on, because Summoners have been taking advantage of the delay associated with activating and executing BPs long before Spellcast became prominent.