Added stress is created by the tires, the point of contact, being farther from the bearing. Hold a sledge hammer pointed straight out, first just behind the head, then by the end of handle. Is one just a little bit easier? Why? It's the same weight?
Rear spacers add stability, no doubt about that, they also apply more load to the bearing. Part of the expense of the Kubota spacers may well be to cover extra warranty repairs. Aftermarket spacers may void the warranty. Maybe not, check with your dealer.
Odds are, a spacer of just an inch or two won't shorten the life of the bearing much. Maybe from 20 years to 18 years, or 40 years to 35 years. Either way, that's beyond warranty. As long as one realizes this going in, I see no problem, just be aware of the possible consequences, there always is one.
As has been mentioned, some have been involved in catastrophic failure of major components.