It will reduce the shock that your motor has to deal with while bouncing around bashing and the shock that your drivetrain will go through from harsh jabs at the throttle and bouncing around but I wouldn't say it's necessary. Very nice to have, but if you don't have the money to spend on it I wouldn't put it very high on the list of must-haves.
All I ever wanted was an honest weeks pay for an honest days work.