Is the inner axel sleeve really necessary?

[Stary Motocyklista_imp]

Tintoy has it right. It is absolutely necessary. Your wheel bearings are not designed to be preloaded. Without the sleeve the full pressure of axle nut torque would applied to the wheel bearings from the inner race, through the ball bearings, to the outer race. This would break them. The spacer makes it possible to torque the axle nut properly to secure the rear axle in the swing arm and not apply preload to the wheel bearings.