[Numpy-discussion] use for missing (ignored) data?

I'm wondering what is the use for the ignored data feature? I can use: A[valid_A_indexes] = whatever to process only the 'non-ignored' portions of A. So at least some simple cases of ignored data are already supported without introducing a new type. OTOH: w = A[valid_A_indexes] will copy A's data, and subsequent use of w[:] = something will not update A. Is this the reason for wanting the ignored data feature?