Peter said: You are describing EAR TRAINING, and there is no question that an accurate tuning device is very helpful with that. As far as performance is concerned, a thereminist should not need a visual prompter to know if the note is on pitch.
It is partially ear training. it is also muscle memory building, and improving awareness of body movement and other hidden variables. essentially, theremin training, as Fred put it. The high resolution tuner in MIDI Merlin can be used to help a theremin player become aware of many different areas of his/her playing. For example: the consistency of the vibrato oscillation; location of vibrato oscillation with respect to the pitch center; the shape of the vibrato oscillation, the transition time when changing from note to note; the degree of pitch error for certain finger/hand/arm motions compared to others; the degree of stillness of the body; accurate measurement of the theremin's pitch range linearity. many of these things are difficult to examine closely with a slow tuner.
I think the challenge is getting over the technical hurdle of using a computer and connecting the theremin via an audio interface. If you download it, try it for yourself, and find that you do not become more aware of aspects in your playing, then you are free to delete it. I have made the tool freely available. It allows anyone to "see" his/her theremin's pitch represented in a real-time with a resolution of 1 cent. It does require putting forth a little effort, and having a tiny bit of enthusiasm to try something new. I realize not everyone is inclined to using computer software, but I'm willing to help anyone that needs help hooking it up. for now, it is still located at this address: http://cycling74.com/project/midi-merlin/
for those theremin players who are set on their methods, or don't feel the 'want/need' to improve, or do not feel they have a capacity to improve their theremin playing, MIDI Merlin would pretty much be useless. I will not be upset if it ends up in the trash can on your Windows or Mac operating system. It all ultimately depends on each person, whether or not he/she is satisfied with the music that he/she creates with the theremin.