Savage, extreme, raids, trials are all designed around an exact "dance" - healers need to heal the raidwide damage at x point, heal the tanks at x point, everyone needs to dodge mechanics because the encounter is built for them not to take extra damage. There's really no challenge in these beyond memorising the steps, which could be faster or slower depending on the difficulty of the content.

Potions would skew that. Suddenly it would be okay that the dragoon decided to eat every single one of Mustadio's right/left handgonne moves (since he'd only be standing in one anyway and one won't kill you) because he could just pop a potion before the energy burst hits. Fights would suddenly be a lot easier to cheese (since right now the only reason they work is because of the challenge of keeping everything going when one person messes up mechanics.) And granted, as a healer, I wouldn't mind the idea that I would no longer have to do clutch healing (ideally I shouldn't be anyway, because everybody should know their steps) but I really hate the level of braindead that would turn most of the content into.

If the fights were less scripted, I could see potions mattering. As it is now, it would simply add a safety net for folks who don't want to learn mechanics.