Guys, as others have said they're supposed to be able to be left outside BUT there WILL be issues if you leave most models in direct rain and sun for extended lengths of time. This is not unique to Kubota...just overall issues.
1. Seat covering will deteriorate and crack
2. Electrical switches, especially on BX models will corrode and fail. I have replaced dozens of these switches on the BX's, even some that were kept inside but maybe got wet during washing or a quick rainstorm.
3. Paint/Plastic will fade
4. light rust may develop on bare metal, nothing significant
Overall covering your machine, even with a $5 tarp from harbor freight is better than nothing and will solve most of the rain and sun issues. If you can swing it, a small metal building, one of those metal carport styles are very affordable (I had a my 18x21 with 7.5' at the lowest point building delivered and installed for $1200).
http://www.tntcarports.com/pricing/index.html
If you can swing buying a $10000+ tractor, add in the extra 10% to properly store it. Since the 12x21 are $1000 and the 18x21 are $1100...you end up with room to store one of your cars for $100 extra.....
I'm not a salesman for TNT, you can buy any brand, just using them as an example because I was so pleased with their product. They run specials on shipping and free sides and such, totally worth it.