middle click is desktop-specific.
a netbook, (sub-)notebook, tablet or touchscreen device would not support that action without an additional modifyer.
using the middle button would also be inconsistent with Gnome's current guidelines, see:
http://library.gnome.org/devel/hig-book/stable/input-mouse.html.en#mouse-buttons
according to the above guidelines, usage of the middle button would in this particular case be addressed to the advanced user only, thereby resulting in being a "hidden treasure".
but there's a discussion going on about how normative these guidelines actually want to be..