FNG,
I just didn't during training for no reason at all. The group I joined, one member is absolutely paranoid about grass and managed to get a mandatory grass checkout into the contract. As I was more concerned about the a/c I would be sharing and as the people I share are, on the whole, actually very nice and easy to get on with people I overlooked this and have yet to get around to the checkout. The a/c in question is an AA5. Apparently the nose wheel is weak and the prop clearance close. The actual practical difficulty in handling them on grass seems to range from no issue to much sucking through teeth depending on who you talk to.
To answer your question, there is no rationale as I see it. Just unqualified opinion.