I wrote a bit about this the other day, and it's been much in my thoughts. This argument is still going on in DIY B&W.
I think this raises some really important ideas about the way we human beings live our lives. The idea of "[b]ut you can much more easily just buy a bottle of rapid fixer"; and "use what you will and trash the rest" represents surely the high water mark of subjection to consumer-capitalist mind control. Not just that one has such lazy wasteful thinking, but that one thinks it has the weight of orthodoxy.
It reminds me of the story about the Soviet rangefinders like the FED2. There was no quality control at the factory, but that didn't matter because the people who bought them were adept enough to be able to sit at the kitchen table with a tool box and tweak them, fresh out of the box. There was colloboration there between the consumer and the worker who manufactured the camera: they were both working class, after all.
Now in our culture everything is pre-packaged, ready mixed, and branded. You can't buy a commodity now, you have to buy a product. I can't get a box of borax anywhere, but supermarket shelves are laden with la-di-da cleaning products with fancy packaging and absurd names. They contain borax, of course, but other things too, that's what makes them a product.
Buying individual chemicals and making up your own developer and fixer just cuts out the parasitic middle person - those who put the stuff together, put it in a box and give it the name "ID11" or "D76". And mark up something like 400% on the chemicals in the process, mind. (The developer's working out at something like 80p a litre, as against £5 for ID11; the sum with the fixer's a bit more complicated, but the differential home-made/shop is in the same region). It's not that the shop price would be beyond my means, it's the principle.