Categorizing music - Is it really necessary?

[Lord Magus]

Artisticly "style" is the label or description applied to the message or method and artist or group of artist conveys, uses, or studies. It is much the same as delineating between Physicists and Chemists, both are scientists, but one focuses on developing, breaking down and understanding the basic elements of matter, and restructuring them to new purposes, while the other studies the effects and interaction of various energies with said matter. While both could easily be pursuing their fields for the pure sake of enlightnment, both their own and others.. others could just as easily by studying chemistry to make money developing pharmacudicals, or studying physics to make money developing space technology. This changes neither's status as being a scientist.
If I were to work in an impressionistic style to convey the sense of light, I would be using an established method for my own edification and further learning. If I worked impresionistic because I know it SELLS, THEN it would be, as you put, "contrived"
Music is the same.. music also has a more immediate catharsis. While visual artists can express and work through their own feelings with their art, music is a more immediate link and outlet for emotion.. this allows others to share in the artists own catharsis.
If the musician thinks to himself "there are always teens looking for angry, fast, heavy music they can vent their frustration to, so I'll write heavy metal music to attract an audience" then the use of style is contrived (in the sense I believe you mean the word).. if the musician instead thinks "I want to express my anger and dissatisfaction, and give others the chance to share my feelings, Heavy metal has always been a style of music that lends itself to what I want to express, so I'll write that kind of music" then the artist is conciously choosing style to emphasise his or her work.
In truth, all artistic endeavors are "contrived" on every level, if they weren't, they wouldn't be "art". Artists look to convey something, be it a story, an emotion, a single image, a look into something's nature. In order to properly convey their messages, artists must consider every aspect of their medium and how it helps or hinders the message they share, from choice of "style", to appearance, to instruments, to font, etc..
Even those merely looking to make a buck "contrive" their content to appease a consumer.
Even those without artistic or financial interest, who paint or sing or act purely for the joy of it, will "contrive" their methods.. from the local bar band that plays 80's music becaus they LIKE 80's music, to the homebody who paints his backyard ala Bob Ross.. they conciously choose and "contrive" their actions to emulate something they value.. be it Glam Rock or Happy trees.