Why would you need a skidplate? Well, the Achilles heel of Kubota's BX range is the plastic cooling fan for the Hydro system. It's a weak $10 part that takes 5 hours to replace. The blades can be broken when driving over branches and other large items littering the ground. A rear skidplate will protect it.
Bro-Tek Skidplate
The "stock" skidplate is manufactured by Bro-Tek - actually they also offer a front skidplate. A skidplate could interfere with the mounting of a Mid-Mount mower, but the Bro-Tek has an opening to permit the PTO shaft to pass through if an MMM is installed.