Thanks for the reply, yes I was thinking the same think. In nature a large lake or pond will do what some refer to as roll over, now I am not completely sure what that means, so I guess it is the bad water being exchanged for good water from top to bottom. In an aquarium a filter like my canister filter serves several proposes, movement, bad water out good water in, cleaning, BB, and gas exchanges at the surface. I can't say whether my canister is doing all I expect it to do, except that my tank is doing great and I am reluctant to change anything. If one keeps their filter good and clean it will perform as it should. In my cansister I have two trays one for sponges and the other for filter floss. When I clean my canister filter I clean the sponges in tank water. I throw out the filter floss and replace it with new filter floss. I do this at least once a month.I haven't read the whole thread. The filter is an important part of your system as it moves water around the tank and allows gas exchange. Without a filter and no movement your tank will become like a stagnant pond. Every aquarium needs water movement. An air stone is a great way to increase the surface area of your tank, but does little else.
I have mentioned in a few threads about quarantining new fish with no mature media, just plants. What I should have added is that I did put a small filter in the tank, filled with filter wool/floss. This served two purposes - it moved the water round the tank, as @itiwhetu said, and the filter wool trapped any particles in the water. If the tank had been set up long enough, some bacteria would have grown but as a quarantine tank it was only set up for a short period.
In most part I am in totally agreement with you about not using bio-media because with a properly planted tank the plants should take the load. But a person and this is the most important part of that equation, should stay well under stocked for a tank setup for plants only taking the load. I believe most people don't do this, and it is kinda bad, but it what it is. I am guilty of that too, especially with plecos....lol...