While I could carry on my mcycle I don't. Even if I could fit a smaller gun in the trunk space (like an LCP) the heat from my exhaust would probably melt the gun to the plastic (it happened to a small can of Honda polish I had in there).
Besides while wearing full one piece leathers, gauntlet type gloves and other necessary accessories it's just not practical to wear a shoulder rig under all that nor would it be easily accessable.
I think about when I am most vulnerable while on the bike, which would probably be while at gas stops out on the ride. I ride exclusively during the day and mostly stop at large gas stations where I can grab a drink and fill up. No hanging out at restaurants, no going to bike nights, no group cruises, no riding on the highway in formation etc. Most likely me and 1 or 2 buddies out carving up the backroads for 200 to 300 miles and then right back home.
If someone wants to jack my bike during the middle of the day while I'm getting gas, they can have it. Not likely anyone will try to chase us down and shoot at us either. For the most part the threat level is low and I see no need to carry while riding. If something does happen, so be it.