I think the idea behind potions/ethers etc. Was to be things used in emergencies that were slow but still faster than kneeling to rest MP or HP.
Some of them do have quite fast use with little penalty. HQ Hi Potions in particular are very good healing for the time it takes to use them, and they do not render your character medicated.
I know back in the day the RDM Maat guides said to bring a Pro-Ether, preferably an HQ one to use because you'd run yourself out of MP even with Convert, given how much spells cost back then and how much you'd need to be doing to Maat.
And I know I used one when I beat him on Dumakulem back in late 2011. The Pro Ether +3 gives 350 MP for a 15 minute medicated status and a 5 second delay on actions after use. But it's a 3 second activation. Not that bad at all.
Regular Ethers +3 restore 40 MP and have no drawbacks other than the 3 seconds it takes to use one and standard item/action delay. Of course, that's drawbacks to your actions...
The main reasons consumables never work out in XI is our lack of inventory space vs. how much you'd need to make it worthwhile vs. the cost of time and resources to have as much as you'd need.
Like buying NQ Ethers from NPCs? They cost like 7k gil to restore 20 MP. Ain't nobody got time for that! They are a level 50 Alchemy recipe as well, and again, if you've got Alchemy 50 you probably wanna be making other things that sell better anyway/are more useful.
The more egregious ones are the Hi-Ethers that take 10 seconds to even use for 50 MP. I'm sure that's faster than old resting, but damn... shitty shitty shitty! Aspir Samba is superior, surely...