I don't like the idea of the air intake for the radiator filling up with rain and snow. That water would have to go somewhere, and eventually it seems to me that it would cause issues. I suppose I could just snap a cover on the top to cover the air inlet and the exhaust flapper.
As mentioned, they do have drains and they do get clogged, at which point water will flow through the radiator, into the bottom of the set, and out a large drain hole in the center of the set.
Before I had mine on castors I had to leave it out for a couple months, I taped every inlet/outlet and the drain hole in the bottom of the set with gorilla tape(I didn't worry about the small drain holes or cracks where panels break, door cracks etc..) and mothballed it, I was afraid mice would get in. It worked well, no moisture buildup, and was mouse-free when I opened it up. The blower motor in my wife's car was packed with mouse nest though!
The large drainhole in the bottom center of the set has(when new) a piece of foam covering it to try and keep mice out, noise in, still let water drain out. The foam gets gone/damaged however, and if missing would be an attractive opening.
Also, on one of the sets I have come across, someone had taken the top panel over the control panel off at some point, and did not silicone it back, they did however silicone the drains closed
and this had flooded all the black boxs.
Even if you don't plan on leaving the set outside, it would be wise to clear all the drains, be sure the control panel has sealant in the right places, in case you have to run in the rain.