I almost always use a Mega potion of intelligence when I do a burst combo on blm. It adds a very nice amount of damage and the duration is enough to get off a second thundara. Vit and mnd are also damage modifiers for war and pld, respectively, so they aren't useless if you use them correctly.

So yes, they are situational. Isn't that what they were designed for? As for potions/ethers, they are helpful while you are solo but elixirs are a lot better to carry around when you are doing end game content.