It certainly dosen't do it any good. I noticed when I went to an enclosed trailer from an open trailer where my equipment was exposed to the elements, I had a lot less problems with the equipment. Things can get tempermental and hard starting when left in the rain.
Yup, here's been my experiences with equipment that's left outside:
1. Over time it "weathers" the starter rope, shortening its life considerably.
2. Same for the fuel line(s), belts, tires, and any rubber components, it's never good for electrical stuff.
3. It certainly doesn't "improve" the looks of the machine over time.
4. If (WHEN) water gets inside the engine via the muffler
I'm sure there are some other things but those are the main ones.
For myself, just covering them with a simple tarp helps.