If that were implemented, it would require a change in the drive addition and replacement code, because it would no longer be a given that the full capacity of the parity drive is indeed zeroes. I don't see that kind of code rewrite happening, mainly because what is there now works, and mucking around with such important code that is currently working would require a VERY strong reason, given the amount of work needed to test for edge cases and general bugs that could be introduced.