"Why do so many cases come with a 300W PSU, when I'm told that this is inadequste for the vast majority of newly built PCs?"
Because it's not inadequate for the vast majority of new PC's! Look at any large scale manufacturer (eg - Compact, Dell, etc) - and I bet you find even smaller PSUs.
Quality counts as well as quantity, and all 300W PSUs are not the same anyway - some have tighter restrictions on the current from ceratn outputs than others.
Now if you're going fro power hungry graphics cards, and multiple hard drives, and lots of lights and fans - then 300w would probably be inadequate. Otherwise it'll do OK.
But having said all that, if you can find a sensibly priced case with a larger supply, then take it - it gves you more scope for future upgrades.