There are very few things that Dick had in either book that are incorrect. This is one that is true; the PO will be functional as long as it is not destroyed or lost. However, I disagree with Dick on this for a variety of reasons, not the least of which is the contaminants that you mention (which is quite considerable, especially if the developer is used warm or hot).

Plus, you ultimately do not know what the FO pt and/or pd salts, restrainers, and other hangers-on will do to your image. What I mean is, develop one in fresh developer, and then develop one in aged developer and you will probably see a difference. Can you quantify the difference? No. You only really know that the fres PO is fresh, and that is a somewhat repeatable starting point for future printing.

This is especially a problem if you use the dichromate method of contrast control, as I believe the dichromate becomes consumed somewhat, by the development, or possibly the buffers react with it, causing the contrast to drift a bit with continued use.

PO is not expensive if you mix it yourself, and it is not an environmentally harmful chemical that I'm aware of, so it makes sense to me to be more agressive about replenishment of the developer. It should help produce more consistant results in the long run.

The most important concern for maintaining the developer is the pH. There was a discussion about that recently here, but generally most people aim for about ph of 5 to 6. As the pH climbs, it becomes less effective as an initial clearing bath. As it crosses over to the basic side, it will actually hinder clearing by converting the FO into a less soluble version.

I us pH strips from Edmunds (100 strips for $1.00) to test. If it is above 6, I add a little oxalic acid to drop it down a bit.