(4e) Sorcerers: is there a reason for NOT using daggers?

[Kurald Galain]

That's not how Double Weapons work in 4e.

Correct. Also, the quarterstaff is not in fact a double weapon (unless you spend a feat on staff fighting).

A sorcerer with two weapons, taking a potion:

Sure, but if you have a disembodied hand, battle harness, quick draw feat, bracers of quickness, or are thri-kreen, then it's much faster. Any of that is rather recommended, because swapping items around is very effective in the hands of an arcane caster.