Good question.
Couple of reasons. In order to get chains on Kubota recommended an 11/2" spacer which they have a part number for, so i figured warranty shouldn't become an issue. I choose to go after market to 2" and I did the front also.(which they don't have a part number for) Normally I would have gone more is better. The wider stance messes with you when your tilling.
And from what i read the 2" and the rim Guard was going to get it. Big difference and I did not want to add four more inches. With the additional 2" we are at about 5" Yes I could use a wider tiller, but the tractor doesn't really have the HP. And we have access to a narrower tiller. So we use a narrow tiller and off set, adjust the three point and it works slick.