I use descending scales in falsetto quite a bit with my young male students (college age) as a means for developing the head voice (with which most of them are quite unacquainted when they first arrive) and it seems to work beautifully. We start in falsetto and then continue down into their normal range; I have them start switching back to regular voice when it's comfortable. My experience with this leads me to believe that using the falsetto aids in keeping them relaxed and open and using the breath properly in a more slender, supported fashion in the high range rather than the straining, push from below feeling most are used to.

Usually the changeover notes are breathy at first, but the student quickly learns how to keep the open, lifted, on-the-breath feel of the falsetto on the higher "normal" notes (usually just below the passagio) and can then continue that feeling to gain the true head voice.

I think that using descending scales is a great part of the exercises; they must learn how to begin each phrase with the capacity for the highest notes so that an appropriate amount of "head" voice and the correct physiology is available for them no matter where they are in their range. (Actually, I use lots of descending scales for everyone at first - along with the "eee" vowel and sirening, it's a great way to get the head voice humming!)