I doubt the shock load on the gear or airframe is particularly significant.
It's true that the runway puts a rearward force on the tire that has to be countered by the axle and the gear. But the force can't be all that great. First, there's only sliding friction between the tire and runway before it's run up, which means that the force imparted to the gear should be significantly less than braking to the point just before the wheels lock. Second, and even more significant, only a fraction of the aircraft weight is on the tires at spin up. This further (and I think extensively further) limits the friction force and the load on the gear.
The wear on the tire is obvious, of course. But it may be cheaper to replace the tires more often than to pay for and haul around a pre-spin mechanism.