You missed my point, which was that a lower concentration PO solution will clear more quickly out of the paper, and will make it possible to possible get a fairly complete clearing of the PO in a single wash bath before moving to clearing baths. The less PO left, the less oxalate to react with any calcium in the clearing baths.

It's also a good reason to encourage the physical removal of the PO by dumping; something that may not be stressed enough in the normal texts.