Regarding Kodak's instructions to periodically discard (in 2-stage fixing), I've never understood why. In my experience, Kodak's info is pretty reliable, but they seem to prefer to err on the conservative (safe for the customer) side.
I remember reading this recommendation in Haist's volumes on photographic processing. The rationale behind it was that fixing can suffer from impurities and processing errors. Note that with a two bath fixer setup every impurity or other problem in bath 1 gets carried over into bath 2 during use, so it will stay in the process forever, unlike in single bath fixing where all used fixer is tossed out regularly. A fresh setup after a certain number of reuses made sure that these issues don't stay around forever and, even more important, don't accumulate up until they become obvious.
Trying to be the best of whatever I am, even if what I am is no good.