Motorcycles are expensive pieces of equipment, many costing as much or more as a new car. Because they run on the same on-road fuels used to cars and trucks, motorcycles are prone to the same fuel-related problems as these cars and trucks. The ethanol fuels embedded in the marketplace give less mileage than conventional gasoline. Riders who store fuel for their cycles in gas cans have to deal with breakdown in fuel quality over time because of phase-separation of the store ethanol-gasoline blends.
The good news for cycle owners is that most modern cycles resist ethanol-softening of seals and gaskets relatively well. But owners of cycles made 10 or more years ago do have reason to be concerned about this particular issue. Ethanol's corrosive nature still does a number on aluminum carburetor parts in bikes.
Ethanol's tendency to pull water from the air and its tendency to dissolve rubber and plastic parts pose the biggest problems for these kind of engines, especially if you tend to leave the fuel in the machine for long periods of time.