In The Burning Crusade, most consumable buffs are classified as either a "Battle Elixir" or "Guardian Elixir". You can only have one Battle Elixir and one Guardian Elixir effect active at once on your character. Battle Elixirs are typically offensive benefits, and Guardian Elixirs are typically defensive benefits. Oddly the magical scrolls like Parchemin d'Agilité IV
are not classified as either Battle or Guardian Elixirs, meaning they can be used alongside your Elixir buffs. Rapidité de Zanza
is a particularly good choice for your Guardian Elixir, since it is one of few Guardian Elixirs that meaningfully increases your damage output. Moving faster means you spend less time running between enemies and more time damaging them and thus deal more total damage.