I don't intend to reignite the dispute, but having spent the afternoon in a seminar discussing the definition of the term 'science' I couldn't help but chip in. Arguably, at the very least, 'scientific' claims should be grounded in precisely measured empirical evidence; and once you introduce subjective value judgements -- like perceptions of improvements in sound and picture quality -- any claims become more contravertible.
Beyond semantic arguments, however, I think it's relatively straightforward: If you're happy with the results using mains cleaning products then that's great. I was sceptical myself, until a Tacima mains conditioner improved the stability of the picture on my plasma and sucked the life out of my receiver. Introducing the Tacima definitely produced changes (for both good and ill).
EDIT: that's just my non-scientific and potentially highly subjective opinion!