The dual wheels were on there when I purchased the tractor. The previous/original owner said he put them on because he had it fall over, yet he didn't add any rop!
I've now got the steel in stock to build a cage, but the weather here is awful, so may not get done for a while.
I have found on soft muddy ground, the 2" (50mm) gap between the wheels easily compacts with mud. I can get a lot out by using the back actor jack legs to lift the wheels off the ground, then running the wheels in top gear to 'fling' the mud out. If its allowed to dry, its a wheel off job to remove the soil.