Yes, but for example WoW had dozens of active skill slots, so one slot used by a potion wouldn´t matter (there was no reason to go into inventory for one). In Guild Wars 2 your healing spell had fixed position and no healing potions - again no reason to go to inventory.
Here we have 8 positions for abilities- both offensive and defensive ones. And if i can simply go into inventory for heal (that doesn´t cost health like Second wind or mana like regular healing spells), why would i slot healing spell into such limited bar. Why would i reduce my nuking potential, when i don´t have to. That´s what I mean.
I know one, Albion. In order to use consumables you have to equip them and you can't change your gear (including consumables) when you are in combat.
This. If in AO you could drink health potions as you wish, duels would be quite different.