Passive vs. Active

[Larmy]

Is there any benefit to staying in Passive Mode while casting?
MP does not regen, and you can't parry.
Is all battle casting intended to be in Active Mode?

[Viridiana]

It's probably not as noticeable when you're soloing, but when you start doing group content, yes, you'll definitely want to be in passive. Your MP can actually regen while you're in battle; it's just that you can't still be casting/moving/taking damage, so you won't see it much solo. If you watch a video of Ifrit from a caster's perspective (like the one in General Discussion about the 8 Santas), you'll see their MP regen throughout the fight.

[DarkArchon]

Passive mode when party is good cuz your MP regen. For solo no. If you're solo, the enemy is targeting you so you cant regen MP. If you can regen MP theres no benefit on staying passive mode, so change to active and use shield and benefit of shield abilities.

[Actus]

Lol @ blocking on THM...

The real benefit is to be able to generate TP for decoy and second wind. Not really much else. Any damage you deal with melee attacks and any damage you might mitigate from shield is minimal and negligable.