Using immersion when low at mana with decent (but not high) mana regen allows you to deal high damage over time without paying attention to your mana pool. Also it has no spell timer.
Mana Sweep fully restores your mana after being casted, so in order to deal more damage you have to quickly empty your mana pool with another spell. And ofc, spell timer, doh.
Imo, relying on Mana Sweep as on your main attacking skill is not really useful. Using it as a supporting skill with another type of sorc makes sense tho.