On topic: I wouldn't have expected people to be against further character development.
Almost everything in Touhou is practically designed for maximum potential for fan contributions. The vague, often volatile personalities are no different.
This means that the fandom has a relatively large role in providing the delicious entertainment, unlike a novel or tv series wherein the content is near-exclusively provided by the author or writers.
For the latter media, character development is great; without it, characters seem shallow and often peripheral.
In this type of series, however, the fandom takes it upon
themselves to flesh out characters, settings and what have you. Each person will ascribe their own interpretations to their works, creating nuances that make the work differ from any other, to whatever degree. When people are looking for new touhou works to enjoy, they can pick and choose from a variety, and if they decide work X doesn't suit them, they can look elsewhere.
If there
was further character development, then there would be a "correct" interpretation. A lot of works wouldn't happen, or would be substantially different, because deviating from definite canonical characterisations and events is often avoided by people, especially the ones who tend to do a lot of research, which are often the ones that produce the most enjoyable fanworks.
People can make their own works as detailed as they like- explaining the minutae of a character's personality, and the mechanics of their powers, as it stands in their setting. Because it's not going against anything from canon, people can freely accept it.
I'd go so far as to say most people who're in the Touhou crowd for the story aren't getting most of it from ZUN's works; he provides the skeleton of works, and the hordes of talented folks out there build their own works from that.
Give people the basics and they make whatever they want; a lot of it is boring, redundant, unimaginitive or poorly structured, but a lot of it is brilliant. Like minecraft.
Give them a detailed, clearly defined product and they'll be interested for a week, then get bored and move on. Like an FPS. Zing!
<Achievement unlocked: semi-coherent sleep-deprived rambling post without proof reading. Aww yeeaah.>