Sometimes I have to remind my players that my DM style isn’t a tournament, nor is there a prize for “most perfect min max combos”. I always prefer style over “substance” (mechanical optimization).
I have an almost allergic reaction to comments like “all Fighters have to use greatswords” or “all Warlocks have to take a dip of class X”.
"All X must Y" claims of this kind are nearly always overblown in the first place, or are a bad, lossy gloss of a much more nuanced thought.
"Greatsword is an especially strong weapon because it gets more out of its Style feat than other large weapons" is accurate. "All Fighters should use greatswords" is
wildly inaccurate. "Blade Warlock benefits
greatly from having their first character level be Fighter, because of style feats and mastery properties" is accurate. "Every Blade Warlock
must have their first level as Fighter" is wildly inaccurate. The only gain with these inaccurate abbreviated thoughts is that they are shorter.
But in noting this, I don't want to give any support to the notion (very slightly implied by what you said here, though I doubt it was your intent to imply it) that one must choose
either style
or substance, and choosing one means forgoing the other. You can have both, and I always strive for both. The two can even feed into each other. You can ask an optimization question (such as "can one character learn all the skills without burning all their ASIs on it?"), which can then lead to an interesting writing exercise ("what kind of story would explain these choices?") Or the other way around, where a flavorful, intriguing character concept drives a question of how to make that idea achieve reasonable effectiveness, like when I went looking for a way to make a PrC I thought was super fun (but mechanically very weak; the Geomancer) actually an
important part of the character, and developed a gestalt Druid/Wizard that could explore all the facets of both divine and arcane magic (which then itself led to a focus on runes and ley lines, things I've always enjoyed.)
Point being, while there's nothing wrong with saying "I'd rather my players make sure they have great style (=RP/backstory/flavor) over great substance (=optimization/effectiveness)", it's quite achievable to pursue both, even if one is a higher priority than the other. To make up fake numbers, you can have a 90 in Substance and a 100 in Style. You aren't working with a limited budget where every point spent on Style is a point you couldn't spend on Substance.