It is not bad to hold the clutch lever in on a motorcycle as long as you are not intending to move the motorcycle. The clutch lever allows you to connect the power from the engine to the rear wheel of the motorcycle. If you keep the lever partially compressed while the motorcycle is moving, this will cause the clutch to slip. This means the clutch plates are not fully compressed and they are spinning against each other. This causes a reduction power transmitted to the rear wheel.
If you continue to ''feather'' the clutch lever when the motorcycle is moving, the spinning clutch plates will get hot, very hot. Over time this can burn the clutch plates which causes the clutch to fail. Typically, you only want to feather the clutch when taking off from a dead stop, or entering a corner too aggressively.