i have always found thumb throttles to be painful after half an hour or so of riding... its also really easy to let off when riding the bumpy stuff. big engine with compression braking and you can sometimes have a nasty experience when the bike suddenly stops!
often used a stubby brake lever for the throttle on my MBs as the supplied twist grips just fall apart... also gets painful but not as bad as thumb throttles...
only thing i like doing with my thumbs (other than twiddling them
) is the gear shifts...
raises a point... it can get cluttered with clutch, brake, gear and other assorted levers... some bars just dont have the room to take an extra lever!