Amey,
EVERYBODY NEEDS A PITCH PREVIEW whether they're partially deaf or not.
Many people, when they first try an audio pitch preview, are disconcerted by the constant tone in their ear and they end up abandoning the preview altogether. They never really give themselves a chance to get used to it. Playing the theremin is like figure skating. No matter how good a skater you are, there is always a chance that when you fly through the air you'll end up on your ass.
Because of the ever-present risk of catastrophe, a thereminist cannot enjoy his/her own performance the way a pianist or violinist can. Unlike all other instruments, the linearity and configuration of your theremin are constantly shifting. The degree of difficulty of what you are doing is so great, and the risk that you will screw up so high, that there is no part of your brain that is available for the pure appreciation of the music. You need all the concentration you can muster just to get it right and stay constantly one step ahead of yourself. If you let go and get lost in the moment, you will end up as flat as the figure skater.
The pitch preview will not take all of the risk out of theremin playing, but it will minimize it. By reducing the general level of anxiety, the preview gives you added confidence and will improve the quality of your playing even in areas where it is of no use. The more skilled you become with the preview, the more useful it is. To begin with, it is only really handy for coming in on the correct note but as you progress, you develop the dexterity and concentration to use it wherever there is a short rest - even one as brief as a quarter note. If you are playing two consecutive notes that are separated by an interval of three octaves, there is no chance that you will get it right, bang on, every time. With a preview, it's easy. (The preview will not help you with connected notes - that is, notes that do not have a space between them).
The olympic skater cannot objectively enjoy his own performance the way the audience can. He is far too busy pacing, watching, counting and listening. Getting sloppy for even one single second means disaster. Ditto for thereminists.
If you are a theremin hobbyist and you are only playing for yourself, then your own satisfaction and enjoyment is all you have to think about. If you are playing for others, then the enjoyment of others is what you have to be concerned about and they may be a lot less forgiving of minor slip-ups than you are yourself.
The pitch preview - NEVER LEAVE HOME WITHOUT IT.