> Very often it's just people who want to have a 7-segment LED display show the number of unread emails, or get a soundcard running, something along these lines.

7-seg LEDs are best controlled by a microcontroller attached via USB or a USB-RS232 interface, as next to no one ships a parallel printer port. Soundcards these days usually all already have kernel drivers, I haven't heard of any modern chip without.

And USB stuff is best developed in userspace with libusb, it's way easier...