The obvious advantage for a caster is that they don't have to rely on spell scrolls, even if they can use them in addition. For others, there are those written spells that even a dumb orc can just read off.
Stealing scrolls with my thief but not being able to use them is absolutely lame. As a first compromise suggestion, you could, for example, halve the intensity i.e. duration for defensive spells and damage for offensive spells cast by scroll or by non-pure caster classes, respectively. Of course, if you want to go right back to the holy trinity, then lockpicking should also be reserved for a thief only....
Maybe we shouldn't get too hung up on this either, since you only have a limited number of actions per round anyway and many things can't be used at all, even if you've hoarded tons of scrolls, vials or special ammo, for example.
With my Battlemaster, which will hopefully become an additional Battle Smith later on, I've only used maneuvers and standard or weapon skills so far. I have hardly used the special ammo so far, the same goes for combo attacks. Whereas combo attacks, e.g. with vials, would already waste the actions of two characters. The class will only be more flexible in the end, which possibilities it can use, but the application of these possibilities itself remains limited and I find that absolutely ok.
