Multiple Sarcasm said:
ending up with a spellcaster unfit for anything other than hiding away in a cave and scrounging for components, let alone a moderately actiony adventure or party support
As others have said I believe you are not seeing what I tried to say. Instead of what you said you actually end with a character learned in many topics: Languages, theology, mythology, arcane lore, herbalism, etc. Why can't she also be the group's diplomat, for example? There have been many historical examples of learned occultists becoming involved in courtly life and politics thanks to their vast education and wise advise, the two easier examples being Gerard Encausse and John Dee, and stories of witches and sorceresses constantly talk about their deceptive, seductive, and manipulative natures.
And why can't she also be the group healer, given her knowledge of herbs? Why can't she also benefit the party with her knowledge of the metaphysical realms and her understanding of the spirit realms, warning them of things not even the party's spy can notice and discovering things otherwise secret like forgotten treasures and the skeletons kept in the closet of their enemies by calling on the powers and offices of demons and spirits, or even the ghosts of the long forgotten victims of their foes? Why can't she warn against things to happen thanks to her knowledge of omens, astrology, divination, and the spirit realms?
Etc, etc. I'm talking Merlin, Gandalf, Morgaine, Suleyman, and the like here and not about a walking piece of very squishy artillery.
Also, take a look at, for example, talismanic magic through history and let's say you have a party of a knight, his squire, and a hunter. What would give a higher benefit to this party, another fighter or a witch who can use talismanic magic to raise their already great skills to superhuman levels, cover their weaknesses, manipulate probability patterns, create magical traps to weaken their enemies, and make her allies both heal and learn faster? And that's just one magical technique or style from among many, all with their own depth and complexities, chosen because it's simple to explain. Think about more complex thingies like invocation (voluntary possession) and the perception altering cappabilities of Glamour, and many others.
Now, and also, go back to the wizard's knowledge and understanding of things outside the realm of common experience: What would that same party do if face against an actual supernatural entity? What knowledge do they have of the weaknesses, strenghts, interests, ambitions, and fears of demons, angels, fairies, the dead, and elemental spirits?
And who else in that party has knowledge of the courtly etiquette of the faery courts, the reputations of the demons and angels they may encounter, and the correct sacrifice to win the help of the river hag that inhabits the currents that go through the old and misty forest they have to
walk prance through?
And good luck next time they don't manage to interrupt the summoning of an infernal duque to earth. What will they do to banish him, delay him, talk their way out such a situation, or even protect themselves of possession? Try to duel a fallen angel that has "only" a couple tens of thousands of years of battle experience and training, as well as extreme supernatural powers?
Of course, most of this is only applicable to low magic settings where the supernatural is actually, you know, supernatural and magic is actually, like, magical, so demons aren't just physical monsters with horns and the like.
Also, how would the knight, his squire, and the hunter deal with explicit puzzles in the lairs of evil creatures, implicit puzzles in ruins of largely ignored cultures, volumes of eldritch lore, encoded diaries, research, etc, by themselves? Punching them on the hopes they start talking?
Finally, who said every single character has to have a direct combat use as their main application? Isn't sacrificing a character slot to open new options, routes, and choices the same as sacrificing levels in a single character game to gain dialogue choices, alternate routes, and extra loot by means of skill checks?