I've got a lot of general curiosity about the scope of duties and the range of experiences among motorcycle couriers in the war. How did one end up doing that? How widespread was their use over the war? Was it ever or often dangerous? If there are any good sources on this (or sources that at least touch upon it), I'd love to see them! :)
An interesting aspect about dispatch riders in WWII - at least British ones - was that many were women. As WWII heated up, more and more men were required on the front lines. Therefore, women [nicknamed the Wrens] started taking up many auxiliary roles, with dispatch being one of these roles. By 1940 they completely took over the dispatch role for the Navy.
It can be a fairly dangerous job at times. For instance, Wren Pamela McGeorge was awarded the British Empire Medal after being hit by a bomb during a dispatch run. While her motorcycle was rendered useless, she was still okay - so she ran the rest of the way to deliver her message.
You can read a brief account of Wren McGeorge's effort here: http://eresources.nlb.gov.sg/newspapers/Digitised/Article/straitstimes19411102-1.2.23.aspx
Generally Wrens were used quite extensively during WWII - for instance, during the invasion of the Low Countries and during the Battle of Britain. They would work eight-hour shifts, couriering messages back and forth between embassies and the Admiralty.
Additional source: The WRNS: A History of the Women's Royal Naval Service