Hang on, hold your horses. What do we know *exactly?*
I doubt your caster level increases with fighter levels. So, a Jaheira won't benefit in Druid Casting by picking Fighter levels. I doubt Astarion gets 4D6 Sneak Damage if he never picks a second level in Rogue.
I think this casting spell level progression is more for those who want to go Wizard / Sorcerer, or Ranger / Druid or somesuch, yes? I *seriously* doubt I get new spell levels if I pick rogue for eleven levels. If not, I'm picking a new class every level; that would make absolutely no sense in the world, nor the nine hells.
Jaheira is by default a druid/sorcerer and thus will benefit from it. Maxbe even doubly and tripley so if she does not fulfill the multiclass requirements and has changed ASI, but that is not sure.
