spontaneous and prepared caster can use staffs differently.
Prepared casters can spend a spell slot to add its level as charges. Ex: They can spend a 3rd level slot adds 3 charges.
Spontaneous caster can reduce the number of charges used by spending a spell slot of equal or higher level. Ex: They can spend a charge and a 3rd level slot to cast a spell of up to 3rd level.
Honestly, the only reason I can see for a spontaneous caster to use that option is because it bypasses the spells known limit. Making staffs for them virtual memory expansions.
For prepared casters, its easy to see the benefit since its spending a single higher level slot more a large increase in lower level slots.
In my view, spontaneous casters have a limited number of spells in their repertoire and it is very hard to change them, but they should have full control over how they use them by spending any slot they want on them. By contrast, Prepared casters should have a lot more choices in their spells each day, but how they use them is more limited (thus keeping the need to prepare the spell at the higher level, and losing the spell when it is cast unless you prepared multiples of it.) If "prepared casters" could freely heighten spells the way the spontaneous casters can AND freely change their memorized spell list every day, then what makes them "prepared", and why would anyone ever choose to play a spontaneous caster?
I think those 2 abilities need to be the divider between the 2 casting types: Do you want to freely change your spells every day OR do you want to freely heighten the limited spells you always know? Pick 1.
Spontaneous caster can only freely heighten their signature spells of which they have at most 9. So the free heightening would improve Sorcs and Bards as well. Also the reason you choose Sorcs is because you have your entire spell-list at hand at all times and you can cast more spells. Given that most spells are situational at best down-right useless at worst having say 35+ spells in your repetoire is prettyu damn good.
it should be noted, that one of the survey questions in the playtest was this very question, and the vast majority of respondents wanted the magic system kept intact.
I can defintely see that. And I understamd Paizo choice. people are a litle wary of "new" stuff even if it way better. Dnd5e left the traditional Vancian and most people approved of it.
I am fairly sure Paizo will release a supplement with a different magical system.
As of now having to learn a spell on several spells lvls as sorc is frustrating (you only have 9 signature spell) and as time goes on more will notice it. SAme thig with Wizards and having to prepare a spell on several slots.
Personally, my favorite mechanic is one where every spell cast tires you some amount, and the more tired you are, the less likely the next spell you cast will succeed. Also, if you have learned a spell (a long process) you know it. No spell slots. No "have you memorized the spell?" No "well, once you cast it, you forget how to cast it."