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.