To me this - the market is not meeting the demand - is the single best argument.

If something is available legally and at a reasonable price (by which I mean not only available second hand for £100 as a collectable, rather than "99p for an MP3, that's outrageous" etc...) I either buy it or accept I won't have it.

If it's not available I have no real issue with getting it illegally.

Frankly if the media companies adopted day and date releases worldwide for content and worked to make their back catalogues widely available through convenient means (by which iTunes, Amazon, Netflix and so on count) a lot of the best arguments in favour of piracy dry up.