Even if somebody creates a thing "for himself", the fact that he knows other people are interested in a similar sort of gameplay or similar evolution of a concept is bound to be an underlying factor in his work.
See, I don't disagree with you, because in actual life there is no such concept as a completely idiosyncratic preference. Even the most idiosyncratic game developer in the world will likely find people who share his taste. Hell, Grimoire has its fans, even on the Codex, and most people aren't as ... unique ... as Cleveland Mark Blakemore, so they're bound to find even more people who'd share their tastes.
And logically, developers would do better to focus on those tastes that are widely shared, than those tastes that aren't, provided they can distinguish between the two. For example, a developer who likes both class based systems and permanent death, would do well to put more emphasis on the former, and less emphasis on the latter, to reach a bigger audience. But I wouldn't fault him either way.
What I would fault a developer for, however, is implementing features they don't care for, only because they THINK other people would like those featuers. Chances are, they're going to get it wrong. The particular example I'm thinking of is Josh Sawyer's implementation of the class system in Pillars of Eternity, which shows a distinct lack of appreciation for class-based systems, and what makes them fun. I blame this poor design on the developer's own stated distaste, which ran against his natural instinct for maximizing customization, which made it into the game in the form of a talents system, which ended up clashing with the game's class roles and making the whole system feel off.
There's a lot more to be said about it, but I don't have the time today. So I'll just say that I don't think, in general, that a developer can deliver a game that rises above mediocrity, unless he is actually passionate about that game, through and through.
You see this not just in system design, but also in story and world design. Avellone is said to have written all the generic companions in Neverwinter Nights 2 - and they're virtually all garbage. But then they gave him time to create and design characters he actually cared about in MoTB, and those were much, much better.
I have no cause to believe that Josh Sawyer is any different.