Thomas, I too wonder what reasonably scientific tests have been done which shows that accordion bottles do not work. It is said that the plastic of which they are made allows air to get in through the plastic. What is the evidence for this? Looking through old darkroom books they were mentioned as the "things to have" and clearly sold well for many years. You'd have thought that in a very short time, if they were defective, they'd not have sold. Even in pre-internet days when magazines like Amateur Photographer and camera clubs were the darkroom enthusiasts' only source of help you would have expected to see complaints but there doesn't seem to have been any
Plastic "soda pop" type bottles are of course cheaper(no cost at all in fact) but can only be squeezed so far.
When I first set up my darkroom, having bought a set of accordion bottles secondhand, I had never heard of forums such as APUG and did not have the benefit of their content. The accordion bottles looked to be a sensible buy and ignorance being bliss I filled them with stock solution developer and never noticed a problem.
In terms of effect we are talking semantics and you wont notice any difference between developer kept in an accordion bottle and any other. However, I would say in terms of Plastic "soda pop" type bottles are of course cheaper(no cost at all in fact) but can only be squeezed so far. When this is the case you just transfer to a smaller soda bottle (cheap as chips).