One thing that gets left out of these types of conversations is the ability and the expectations of the player. Most folks dabble, some get pretty good, a very few become world class. I can tell you absolutely that looking at your hands when touch-typing is a bad habit because in the end it will slow you down. But if you don't type a lot and never will, I guess who cares? Are tuners and/or pitch preview genuine enhancements, or crutches that will somehow ultimately hold very good players back?
1. In general, and due to a lack of any pitch reference, a raw Theremin isn't well suited to a cappaella performance.
2. A well designed pattern-based real-time pitch display can help you play and improvise by showing you where you are in the scale. It can teach you a lot about intervals and melodic structure.
3. Real-time pitch correction is inextricably tied to absolute pitch, so you need some sort of reference to use it productively rather than fighting it. That reference can be your accompaniment, some sort of drone, or a real-time pitch display.
4. Can playing with no references cause bad habits? E.g. might your technique suffer from having to hover right at the edge of audibility for new notes?
I think playing aids get looked at too philosophically, or as cheating, or as the source of bad habits. I'm a dabbler with low expectations so I'll gladly take all the help I can get. If I never get really good on the thing it won't come as a surprise and I'll have plenty of company. In the extremely unlikely event that I start to become really good (requires time and effort I'm not putting into it) I'll gladly cross that bridge when it comes and try my best to unlearn any bad habits then. It's no fun to unlearn and relearn something ingrained, but we end up doing it all the time for other stuff, and it's usually not the end of the world.