Just found a link to a report by the CAA about the use of portable electronic devices (PEDs) and the interference they could cause:
http://www.caa.co.uk/docs/33/CAP756.PDF
I will admit I haven't read it all yet (busy time at university and then even busier time in the pub!), but it does seem to offer a reasonable explanation as to why airlines follow the procedures other members have talked about.