Well, actually, it's not strictly true that you need a computer to get CD content onto a flash or HDD player, but clearly being able to temporarily use new content (in uncompressed commercial or home-made audio cd form) with tech hassles is better for simplicity.
If you could borrow use of a CD player, on your travels, and had a record capable HDD or Flash deck, you could connect line-out->line-in and real-time direct record... to mp3 commonly, and either record to one big complete disc mp3 track or manually play/record one track at a time.
But that's kinda adding a complication to the extent it's quicker to borrow a computer than go through that routine.
And since a traveller's friend is simplicity, that recording/encoding of new content is kinda stepping outside of the keep-it-simple ethic
