There's isn't really a rule that magic has to be prepped, which wouldn't be suitable for character's of mine like Solomon and Anna for example, whose only offensive capabilities fall under the sphere of 'magic.'

More conventional attacks like fireballs and the like benefit from being charged, because they can't be defended from without an equally prepared defense. I assume the same applies for defensive magics against non-charged physical attacks, if it's logical.

That's just my take on it though, there's a lot of room for interpretation in T1, especially at levels above t2-t3.