Yeah and in my fight with Omega Weapon, I nearly did run out of all healing items. The battle essentially did become a matter of how many you'd bought ahead of time (and how much advanced magic you'd created ahead of time).

The point just being that you could use them all instantly, so unless an attack kills all 3 characters at once, you can almost definitely survive it. FF7R fixed that by having the ATB gauge so you can't just revive 2 characters instantly.