It is my semi-learned opinion that if the blade flexes down a given distance, either by a meaty hand pulling, or by an inertial force during a tow across a rough field, the stress on the mechanism is the same.
Of course, the limitation on routine pulling of blades, once per pre-flight inspection could be based on the many repeated cycles of damage the blade mechanism would accrue. OTOH, I have heard of some homebuilts where the blades must be supported in a crutch for tow operations due to these stresses.