I've been using one of these: http://www.cjemicros.co.uk/micros/indiv ... =VAR-35LCD
It has proved very useful to have it plugged in whilst I'm trying to test various DVI Monitors or HDMI to VGA adaptors. Sometimes the R-Pi is not recognising having anything connected to the HDMI so it defaults to Composite output. I can (with my glasses on!) just use nano to edit the config.txt file to change monitor settings, which saves having to power down the Pi and move the SD card to a PC.
I think the main use for small Composite screens will be when a Pi is used for control/monitoring applications such heating control when a small amount of info is displayed in large text/graphics.