I agree with Blue Jays & others that one can shift up without the clutch, but the clutches these days are so easy to operate that it is just a light flick on the lever & you are into the next gear.
You will hear about how you can shift down as well, but that is a different story & a LOT of people trying it really screw up the poor old cog box.
Obviously if those racing be it dirt form or road racing have a clutch lever on the bike there MUST be a reason for they are using them continually.
True I can take you back in history of where we had to stiffen up the clutch levers of any bikes being raced & after a few laps the strain on the hand besides handing onto the bucking machine was simply to much & we simply "crashed the box" meaning up & down shifting without use of the clutch. One of the reason why I was continually rebuilding my cog box & those of fellow competitors in our m/c shop.
Some yrs back I had this lovely little Yamaha TZR-250 with far more TZ parts with it then normal. My left wrist was broken again while practicing Observed Trials up in the dirt. Rod the TZR-250 down to local hospital (when we had a hospital) for X-rays then rode the TZR-250 in Penticton Hospital with X-rays in my jacket chest area. Specialist is putting on cast & then in a sort of asking question he said "Did you ride your m/c with the X-rays?" The answer was YES so asked how I was going to ride back, so said something like "just leave me a thumb & one finger or possible a sort of hook for the lights & stop signs. Made it home to S'land with fore-finger & a thumb, but only used at stopping & take offs for shifting up & down was done without the clutch------still one has to know how that is done otherwise all hell can break out.