ILYA,
Ever since the theremin was introduced, the community of musicians drawn to the instrument seems to have divided itself into two distinct groups: those interested primarily in aleatoric "Free Music" (like composer Percy Grainger) and traditional musicians (like the late Clara Rockmore) who devoted themselves to playing traditional melodies with accuracy and precision (hence the expression "precision thereminist").
The difference between aleatoric (= chance) theremin playing and precision playing, has to do with the ability of the thereminist to accurately repeat what he or she has just played so that for all intents and purposes it is identical to previous performances of the same composition.
A third category of theremin playing introduced itself in the late 1940's when Samuel Hoffman began using his RCA theremin to create effects for SciFi, horror and suspense film soundtracks. Hoffman was a classically trained violinist and, like Clara Rockmore, he brought his skill as a professional violinist to his theremin playing. Although Hoffman played FX theremin for dozens of films, as far as I know he had no interest in aleatoric or Free Music.
There are plenty of thereminists who play exclusively experimental music, and they do very well at it, but they are unable to play THE SWAN or the VOCALISE or any other theremin potboiler with consistent precision. I would not call them "bad" thereminists or "bad" musicians. They are simply not precision players. As one experimental thereminist said when he was asked if he could play THE SWAN, "Why would I want to?"
As far as I know, there is no such thing as a "non-precision" or "aleatoric" organist, violinist or trombonist. The reason for this is that although you could play aleatoric music on any one of these instruments, in order to do it you would have to possess a level of technique that would permit it.
With the theremin, as RCA pointed out back in 1929, no skill or musical training of any kind is needed and "a child...an elderly lady....a blind man" can do it without any practice, knowledge or musical instruction of any kind.
Amey - "...it's nice to have you back where you belong!"