Our tractor lives outside. It has not suffered any major problems from this life. The other thread had some interesting comments ... but on I will comment on ...
Things last so much longer with a roof over them...uv is a biggie, and any crud that falls on the machine(needles, leaves and such) seems to hold water against the finish. A little lumber and a Saturday worth of time will go a long way in keeping your new machine looking lovely.
It may be true that a roof would be nice and one can be built in short order ... however, it is not as simple as a "Saturday worth of time". Around here any flat (or slanted/peaked/or anything that could be defined by the County as a roofor deck) that is more then 32" off the ground requires a permit!
To get the permit, you must submit an engineering plan complete with load rates and wind sheer calculations. These formulas are nearly impossible to do yourself - you need to be an engineer or an architect.
So, perhaps your friend/neighbor has already built one ... just borrow his plans and submit them ... Not So Fast ... they put a stop to that too because each plan must have an ORIGINAL signature on it. The engineer/architect will charge you the same for that signature as he will to draw a new plan.
If you get caught with the roof up and no permit you can just pay the fine and it will be OK ... NOT ... they can fine you, make you pay for the permit you should have had and
make you tear it down. Then make you submit the paperwork for a new permit complete with the fee for the permit ... then they can "hold the permit" before issuing it or refuse to issue if they feel like it.
So ... our tractor lives outside and works just fine.