This is a funny thread, with my Blade I bring my jacket, gloves and my helmet with me, never leaving them on my bike - total cost - maybe $600 for the lot...
On my Harley, I leave my lid, gloves, leather jacket, chaps, boots - probably $1200 - all under or on my bike...I have never worried about anyone stealing them because what kind of idiot is going to go up to a group of Harley's and start lifting gear, really you would have to be a real dumba$$.
With that said, I don't generally ride my sportbike with anyone else there fore no group of bikes, just mine...easy pickins. If I was with a group of sportbikes I would probably leave it there for the same reason as above.
If I lived near a city I would always lock mine up, regardless of which bike I was riding, but I live in the country in Vermont where we all carry guns due to no carry laws to speak of...again, pretty stupid to go picking up stuff that isn't yours.
my .02