1. Because even a powered rudder stand a chance of vibrating in flight if un-balanced. Ref B767 AMM:
FLIGHT CONTROL SURFACES THAT HAVE AN
INCORRECT OPERATIONAL BALANCE MOMENT AN
/ ZGGG), China">CAN
SHAKE DURING FLIGHT. REFER TO
THE STRUCTURAL REPAIR MANUAL ( SRM 51-60-00) FOR
THE APPLICABLE OPERATIONAL BALANCE MOMENT DATA.
You can balance the rudder on or off the aircraft depending on how far out of balance it is. The only way to balance, according to the above SRM ref is as follows:
. Serviceability can be regained by removing the paint buildup from Repaints and Repairs that were performed without paint removal, and refinishing the rudder.
In a nutshell, the SRM tells you to maintain a record of repair. Keeping track of material removed and installed. Using simple math, determine what the new balance moment is. If it falls within the limit; do nothing. Between the lower and upper limit; remove paint. Above the upper limit, remove the rudder and re-balance. In fact the SRM says:
Ask The Boeing Company to analyze the Rework Record to determine the rudder's further serviceability.
I try not to use the actual limits in my replies, so as not to confuse. Always check the applicable SRM or AMM. (Standard disclosure)