Like the Sorceror, they can use spell completion items. So, they can still use scrolls and wands for spells on their class list that they may not otherwise be able to cast.
Also don't forget Multiple Attribute Dependancy. Favored Souls and Spirit Shaman use Charisma to determine what level of spells they can cast and how many bonus spells they get. They use Wisdom to determine spell DC's. You can kind of ignore the MAD if you just take buff spells and healing as the Favored Soul. The Spirit Shaman takes a pretty solid hit if it skimps on wisdom since many druid spells are good for blasting.