Well, let's take a look at how the PT is constructed VS a normal tractor.
PT has a hydraulic cooler (similar to a radiator) right on top of the hood. Water will rain down through that radiator and directly onto the engine.
A conventional tractor has a full hood over the engine, protecting it from rain.
PT has a semi-horizontal dash board where the keys and switches are also exposed to any rainfall that comes in under the canopy.
Most farm tractors that I have dealt with had the key and switches mounted on a vertical panel, under a drip tray, that would channel rainwater away from them.
Those two things, rain on the engine and keyswitch, are the two things that would make me tarp it if left outdoors.
Let's face it, the sheet metal is not sheet metal. It is plate steel and is crazy thick. It will last for eternity and, although it gets surface rust when scratched, I just take a grinder to it, spray on a little primer and JD green and it is good as new.