I used to wonder as well, but after experiencing Ragnarok difficulty on union raid, I do think that it is more beneficial to get attack power than max HP.
Part of the reasoning is that with higher attack power, its easier for you to deplete the boss' overdrive meter and giving you control on how you sustain your unit with cooldowns. I dont think it's viable to rely on your healing skill against most boss' overdrives.
If you can send the boss from rage to stun in 1/2 turns, then it is less likely for you to take overdrive from the boss... Also for some bosses who buff themselves with their overdrive, you can control their buff and instead of relying on 8 turn dispel, you can send them to stun when they are about to use their overdrive instead. With limited source of dispel (so far I only found one in Cassiopeia), its easier to control their rage/stun meter than to take and sustain your team from overdrive after overdrive.
Obviously balance will be important, no point having high attack if your hime is dead, and obviously there's no point in having lots of HP and your hime deal little damage to the boss allowing the boss to overdrive you over and over.