Makes more intuitive sense to me that the linkage would take excess stress, from an unlocked helm.
Well, here are my thoughts on this, and it's just my opinion, based on inspecting / repairing a fair number of rudders over the years.
The steering linkage will absorb some energy either way, but bearings tend to wear due to movement (which increases with load and torque), rather than deformation due to stress.
The rudder will rotate, even with the wheel locked, but only to the extent of the steering mechanism backlash.
If the wheel never moves when unlocked, it will be the same difference with it locked.
However, some move rather violently when the boat is rocked by waves or wake.
It is more pronounced in lighter boats, however, these are likely lighter built as well, than their passage-making counterparts that tend to be much more stiff.
Also, it depends on how well the rudder is balanced hydrodynamically and the inherent resistance in the steering.
Remember that most vessels are not underway 95-99% of their lives, and even then, sailed quite conservatively.
Because rudder bearings on these infrequently and lightly used vessels do wear out, I conclude it is not due only to underway movement, as if that were the case, a sistership would wear out the rudder bearing in just one ocean crossing, and they generally don't.