Since a majority of the bikes in around 1975 or so were left side shift that was the direction the DOT went, left shift/left clutch/right hand brake/right foot brake/right hand throttle.
The arguement for right hand shift is it sort of balances actions and uses both sides of the brain for each function, braking and shifting.
Having ridden both, starting on right hand shift Bultacos I like the right side shift. Almost got me in trouble when my brother and I traded trials bikes, my Sherpa T for his Yamaha TY250. I'm going over a drop off trying to brake with the shift lever - not too functional. Still, if you ride both a lot it becomes an automatic function within a short time.
The real fact is it doesn't probably make any difference when one is used to a bike. The good part is the universal nature of all bikes for road use having the same layout - well at least all bikes made since around 1975.
It's a push... a draw, a tie.
KLX650C, Zephyr 550
SR500, Bultaco Sherpa T