I am just thinking here...
It seems that balancing the fan would not have any affect on the fan assembly itself...as you are not modifying the assembly, but rather just compensating for their imbalance at the bolts. So the blades and rivets would seem to be a simple matter of fatigue, from flaws or later damage...but not likely from the imbalance. In other words, the blades have no idea wheather the balance weights are on or not. They will still be imbalanced after the weights are added, the weights just compensate for their imbalance.
The balancing would really only seem to matter to the hub extension and the front of the crankshaft...as they will receive all the stress from an imbalance. If the fan is balanced, the hub extension will see zero extra load. If the fan is unbalanced, the hub sees all the extra load...and the crank sees even more, as the extension acts like a lever.
Anyway, like I said...just trying to think it through here. I think the balancing is for the crank longevity, and not for the fan assembly.
John