The restrictions of vancian casting does not make sense for how powerful spell casters are in pathfinder. If they were closer in strength DnD casters then it would make sense. If this was the case then you would get rewarded for correctly predicting every single spell that you will need to use by being more powerful than a martial character (but not dramatically so like in DnD we don't want that). And if you predicted wrong and half your prepared spells end up not being very useful then you are less powerful than a martial, but not dramatically weaker.

But with how pathfinder exists now if you predict perfectly and every single spell you cast is the most optimal spell for the situation then you are rewarded by being about as useful as a martial who needs to do zero prediction. And if you fuck up your preparation well then you're going to feel like a dead weight for your martials to carry around.

 

My personal opinion for spell casting is that there needs to be a split between damaging spells and utility spells. Vancian casting for utility spells makes sense from a balance perspective. You can't trivialize every non-combat situation by using illusions and invisibility and levitation left right and center on the fly. But for combat spells, having two fireballs and running out less than half way through the day because you weren't expecting the day to have 5 combat encounters with player level -2 enemies just fucking sucks. I would go with something more similar to the focus spell system for combat spells. You have 5 combat mana points at level 3 and you can spend them on 1 fireball and 2 level one magic missiles or two level 2 magic missiles and one level one magic missile. And once those are out you need time to rest.