I have only grown two "deciduous" phals - honghensis (x2) and parishii.
The parishii is watered daily and has never lost all of its leaves.
The two honghensis plants were side-by-side in my greenhouse; one was watered daily, the other not. Guess which one lost all of its leaves when the other didn't...
Based upon that anecdotal evidence, I view them as being tolerant of being leafless, not as requiring it. (But with such a small sampling, I might be totally wrong, too.)
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