why is it preferable to have a situation where statics want to avoid a second physical ranged but are willing to take a second caster?
Second caster OR melee is a lot more possible jobs than second ranged. That's why it's preferable. Of course, it'd be even better if ranged were behind by just enough to make it an interesting tradeoff - I'm not arguing on that level of detail here. It's just that as a binary question of "should ranged be just as strong damage/defense wise as every other job", the answer is no.