There are advantages like larger axle, larger bearings,different sprocket sizes easily changed, and easier to install a disc brake rotor.
Yeah but that's most of the things that are needed with motors that make a lot of power, need both rear wheels to spin at the same speed, high speed turns, and different track sizes like in karting.
None of us minibikers need disc brakes in Southern Cali. Most get them at a very cheap deal, or just want the look of the disc setup and have the money to spend. We all ride, or race in straight lines so we can stop by using are feet if needed too.
Larger axle, larger bearings, bearing hangers, axle locks, disc brakes, larger sprockets all add up to more weight. If your dealing with less than 20 hp it isn't needed.
If your twisting and turning, or normally ride on wet conditions then I can see the need for disc brakes.