This is a standard feature of multipowers, or it used to be. There's no reason you have to buy a multipower slot to the full AP available in the pool--but it's usually stupid not to since you're getting an 80% or 90% discount on the slot.
There also used to be a limit to the number of spells a caster could have running at once, typically INT/5. Again this rarely matters since you rarely need to have more than attack, defense, and movement going at any particular time, and people rarely play stupid spellcasters.