Thesis: Careerism helps a composer write better music.
Argument: A careerist composer finds ways to promote themselves and their music. Through such promotion, their music gets performed more frequently. Hearing one's music performed is one of the best ways to learn and grow as a composer -- that is, to learn to write better music.
The argument could afford to be fleshed out more, especially the last sentence. And exactly what is meant by writing "better" music can be a huge point of contention. But, at its basic level logic is pretty sound.
So often "careerism" is thrown around as an negative thing, especially among composers. For example, a review (by Allen Gimbel of the American Record Guide) of a recording of John Corigliano's Circus Maximus and Gazebo Dances states, "[the Gazebo Dances] serve to remind us of this composer's youthful genius before careerist distractions set in." But what, exactly, is so distracting about careerism? I suppose if your idea of a career in composition is to get a professorship somewhere so you don't have to care whether your music gets played, then maybe this would detract from the quality of the music-making, but seriously, who does that?
There's also the notion of the careerist composer who writes works specifically designed for audience appeal rather than artistic merit, or who capitalizes on the success of an early work by churning out countless other works in imitation of that breakthrough work. And I suppose there are composers who work like that, though they will generally tell you they write only to satisfy themselves or some similar nonsense. But even in that case, if that is your musical goal, then certainly careerism will advance your goal.
But even if your goal is simply to write music that is the best expression of your artistic ideals that you can muster, a little careerism can go a long way. As long as you do not lose sight of your musical ideals, self-promotion can only help. Obviously it will help your career, and if you are dependent on your music to make a living, then this is a basic necessity. But it will also help your music, in the long run.
Now, the idea that hearing your music performed helps you to grow as a composer may be debated. Certainly there are counterexamples in the form of composers who wrote wonderful music without hearing much of it performed -- Ives and Scelsi both come to mind. To be fair, Ives and Scelsi have both had their share of detractors, decrying their music as amateurish, but I think each of them has left an enduring legacy of important works displaying a great deal of originality and creativity. Perhaps if your musical ideas are as revolutionary as theirs, you too can afford to languish in obscurity for most of your musical career (provided as well that you are independently wealthy, whether by virtue of your success as an insurance executive or your station in Italian nobility). But in general, learning to effectively channel your creative impulses into a finished composition takes a lifetime of work, and there is much to be learned from hearing your own work. Thus, a healthy dose of careerism will improve your musical output.
This is all well and good, but often I wish it were not so. I'm just not good at promoting myself, for a variety of reasons. In college, my preference for hanging out with the math nerds meant that I didn't form useful contacts among my fellow musicians, one of the first steps in building a network. My social anxiety makes it excruciatingly difficult to even send an email to an unfamiliar director, and working by phone is harder still. And for a long time -- even to this day, to a degree -- I clung to the idea that if my music was good enough, it would get out into the world without my having to work at it. As a result, my musical career has had a hard time getting off the ground, and what's more, my music has suffered. I don't mean to say that it has gotten worse -- I consider myself a better composer than I was five years ago -- but I haven't developed my skills as well as I might have. I'm trying to make improvements, both musically and professionally, but I have quite a way to go, and when I compare myself to other composers near my age, it often feels like I'll never catch up. Building a career won't be easy, but it will definitely be worth it.