My use of microintervals is not based on any theory. My reasons are practical:
musicality, expression, experience. Almost all the microintervals that I
have used I had heard before, either in some "natural" music, such
as ethnic music, or in the environment. I have worked with them in instrumental
music and occasionally vocal music, using my own aural experience (no electronic
tuning devices), respecting the natural limits of performers and their instruments,
and sometimes even taking advantage of those limits. While composing I have
relied upon my voice, the guitar (bending the strings as well as retuning
them), and occasionally recorders (using incomplete covering of the holes).
My idea has been to enrich the music with natural elements (just as a tree
never grows geometrically straight), but not to make it more "sophisticated."
An outline of my different approaches, sources and reasons for microtonality:
Composer's comments for Walden, the Distiller of Celestial Dews
(2000) for mixed choir:
Rain, a window, roofs, chimneys, pigeons and so on and railway bridges, too (1992) for 18 instruments