Some time ago there was a very interesting article in Trail magazine effectively advocating that we should use standard fabric conditioner instead of (very) expensive proofing agents for our breathable fabrics.
Recently I put this to a salesman in a very well know Keswick gear shop (famous for it's quality CS), he became quite defensive, suggesting that the fabric conditioner would eventually break down and destroy my Gore-tex jacket, he also rubbished other proofers other than the only type sold in the shop.
A likely story I think, any thoughts or new research I haven't seen yet?