Randy,
The Twin has always been hangared but also sits outside when on the road. It got sponsored with new windows (single piece windshield etc) in 1993 and most folks say they still look new. If not for the sponsorship and the one-piece windshield, I'm sure we'd still have the originals.
The big deal is no one cleans them but me since 1996. There is a placard provided by AeroPlastics that says "Do Not clean windows and No fuel needed" Picked it up at SnF awhile back.
I use lots of water to lower scratches - distilled or rain water becasue our local water has lots of minerals in it and my hand sans rings to get bugs off. Wiping is done with 100% cotton worn-soft T-shirts cut up.
If yours are already crazed you might try the Meguiars products to get them clearer. Their products worked well on my Miata plastic rear window in the convertible top
http://www.meguiars.com/internationalLiving in TX, if the plane is on the ramp we have interior very thin, very light plane shields that velcro in the windows to keep the heat lower and protect the avionics - and if you are camping, nice for privacy to change clothes. They are metallic on the outside and white on the inside. I've tried to get another set but the mfg seems to no longer be around.