A skilled theremin player is able to control pitch, dynamics, vibrato, and portamento with only two hands. To (accurately) recreate a theremin part on the synthesizer, you would need one person twiddling knobs while another plays the part, and they have to coordinate their movements.
Trust me - I have two monophonic synthesizers and a theremin.
Check out Clara Rockmore and Theremin's niece (forget her name offhand) to hear what a theremin is really capable of. A bitch of an instrument to play well.