Full disclaimer: I haven't played the Deadfire beta, so no hands-on experience there.
Anyways, using POE1 as a base, I'm having tons of problems understanding people that find Multiclasses underpowered because of "high level abilities". In most cases this doesn't seem influenced by actual experience playing the game, but rather because it "sounds" like they're going to play an inferior character. There's tons of people who've complained about it ever since Josh's explained the outdated version of multiclass so it really seems like a conceptual gripe.
The thing is, in the original game a higher-leveled ability didn't ALWAYS meant more powerful or useful. Not even as a caster. Do Wizards cast all of their highest leveled spells until they run out, followed by the second highest level until they run out of charges, followed by the third highest level etc? They certainly don't. I'd even say the low to mid levels of abilities in Pillars are spoiled for choice and utility, while the highest levels are often not quite as awesome or game-changing. Design-wise this only makes sense as you shouldn't wait until max level for the class' identity to suddenly appear.
A multiclass character in a game like Pillars is de facto useful because having more abilities and options is de facto good in the game. With multiclass Rogues can now cast group buffs. Wizards can be party healers. Fighters can have pets at their side. Monks can use chants. You get to play the whole game with one or more character like that. At that point... does it really matter that much that your character doesn't get the Green Dragon Punch of Destiny at max level? Opinions may vary but I honestly don't think it does.