A lot will also depend on how you use the reader. Looking inside the case for mine there are cards containing:

1) Game walkthrus and maps. (Not for me. I never need help. They're for...a friend. Ya, a friend.)

2) Various programming sheets, articles and books for CAD and tooling work. (Actually used to make working for the man a bit more efficient and keep the work area cleaner.)

3) Maps. (Don't even ask.)

4) Every hardware and software manual that came in an digital format (including RTF, PDF and, after a little converting, CHM). Anyone with a small apartment knows how easy it is to lose docs. Especially docs for something a couple years old. This also includes printed website forums since often those contain better info than official product documentation.

5) Ummm, a card with the chant "You do not want to go back to jail. Let it go." with some nice soothing music under it. (Usually used only in extreme situations. Useful when people won't let you read in peace and insist on telling you all about there current fungal problems and, oh hey, what are you listening to?)

All of it would fall in the category of "like to have available but do not need immediately". Cards are perfect for that.