The switch can act as a second life for your headphone –if one of the two signal pathways or solder contacts fail between it and the driver– one flick away. Other than that it crowds your cups like a rat's nest with wire, that's why the HP2 is considered “purer” sounding... the HP1 could be considered better “damped”.
(From the official HP 1000 promotion brochure, written by Joe)
I only go by what Joe told me. Can't confirm this with him now. :-(