I assume the reason for leaving some solution and not draining it all is a safety measure in case of some failure. Is this correct? If so, I guess this is system dependent. That is, if roots or wicking material cannot reach the remaining solution, no safety is created.

Do beds really need to be deeper than X inches with constant cycling for egg plant, tomatoes, zucchini, etc? That is, what is X in your experience/opinion? How much depth do you leave after Bell siphon drainage?