1st: The only potions people use are haste and mana, outside of that, potions are never used.
2: Most everyone I know uses Battle/Guardian elixirs instead of flasks. Its generally better.
3: Elixir Master is basically also “Flask Master”, you can proc extra flasks from elixir master, so even if you prefer flasks, elixir master is the correct choice.
I’m an elixir master and do not regret it one bit.
Armor potions are used by tanks. Destruction potions are used by some casters. The type of mana potion also varies - Hunters/Druids use Fel Mana while Alchemists use Mad Alchemist’s Potion, Warlocks use Rejuv and most others use standard mana potions.
It’s better for healers and for Survival Hunters. Everyone else tends to use Flasks because they’re more powerful than Battle Elixirs and Guardian Elixirs are generally terrible.