IMNSHO the theremin is not appropriate for music therapy.
It is great as a musical toy, an afternoon of sonic fun and discovery, especially for kids, but as a genuine tool for “therapy” (which means HEALING) it doesn’t work. I used to think that the theremin would be ideal in a therapeutic/musical context but that was back when I was madly in love with the instrument and blind to its flaws and limitations.
Below is a link to a picture of me after pitching the theremin to a convention of physiotherapists in Montreal back in 1998. At the time, I thought the theremin would be the perfect instrument for healing of different sorts and I did a number of enthusiastic presentations to professionals.
I won’t bore everyone with the gory details, but the general consensus among qualified people was that there was a frustration factor with the theremin that precluded its effectiveness as a healing tool. Yes, it is probably the easiest musical instrument on which to make a noise, but it is the most difficult of instruments on which to produce the kind of harmonious and acoustically satisfying sound that is essential for the production of the kind of calm serenity and harmony needed for healing.
The theremin, as fans of SciFi and suspense films know, is disturbing! There are all sorts of other musical instruments, both acoustic and electronic, that can supply genuinely therapeutic sounds that are easy to play and that are capable of polyphony as well.
http://www.peterpringle.com/pppix/aftercon2.jpeg