We may also divide music into fun levels and serious, soul feeding levels. For example, ragtime can be really catchy and fun, but does it suggest deep metaphysical connections? Early rock n roll is fun for me, but not suggestive of depth. Of course, my guess is that not everyone wants to hear depth in music---maybe some find it depressing or even existentially terrifying.
Certain forms of music can have a mathematical aspect, which can be especially appealing to those of us inclined toward math appreciation. Indeed, the opposite could be claimed, namely that to some of us mathematics itself can have an aesthetic component. Equations and proofs can strike some of us a very beautiful, and can excite wonder. But admittedly the person for whom this is the case is probably in the minority. The music of Bach comes to mind here---indeed, I believe that late in his life Bach did join a mathematical society, and provided a fugue or a round as his requisite mathematical work.
I think another level involved in appreciating music is just an appreciation of the level of virtuosity required to play it. Classical piano has an obvious athletic appeal when certain spectacularly difficult and fast pieces are played, such as “the Bumble Boogie”, many of Chopin’s Polonaises, or Liszt’s virtuoso pieces. Or think of Paganini’s music for the violin, which appears almost impossibly difficult. Of course, a great many other virtuoso showpieces by numerous composers could be mentioned. But are such pieces reaching us emotionally and in terms of pure music?
Yet another consideration is whether music, to be appreciated in some ways or at some levels, needs to be uplifting or healing. In a way that is analogous to tragedy in the theater arts, perhaps dark “depressing” music has an essential place in the musical palette of some of us. Of course, it can be argued that while such music is superficially a “downer”, it may on the whole have a positive effect on the listener that seeks it out, just as tragedy does in literature and the other s arts.
To return to one the original questions about what kinds of music are useful in “music therapy”, I think all of the above considerations tend to show that the topic is very complex. In particular, people who merely assume without much thought that “music is healing”, or that “music is uplifting”, have probably not thought through all of the issues. And I suggest that a “one size fits all” attitude to the music therapy issue is as ridiculous as assuming that one particular medicine would cure all the various forms of sickness and ailments that human being can have. Just as in the case with any treatment--medicinal, surgical, or otherwise---the music that might help a given patient will depend on many factors, particularly that individuals tastes, as well as the patients mood, context, and condition at a given time.
Me, I find music indispensable, and certain forms bring enormous joy to my life. But by no means do I love “all kinds of music”, and I strongly suspect that few, if any, really do, in spite of the frequently heard claim.
To sum up, the appreciation of music is a highly subjective and complicated thing. For some of us, music represents the noblest and holiest art form available to us in this world. It can obsess us, and seem to be something we cannot imagine living without. For others, it seems to be primarily a form of pleasantry, something to provide an ambience, or a badge of showing what social group we identify with. Music therapy is likely; it seems to me, to be most effective with the former type.