WHY you want to go on rides? To enjoy the wildlife, the scenery? Or to race? That seems to be the questions to me.
Most times I REFUSE to go out with people I know (most with RZRs and Can-am Mavericks) because their idea of a good ride is to drive as fast as possible over rough terrain. Pretend they are in the Baja 1000 or other off-road race. They don't care about the scenery or that herd of elk they just blew by. Me - I ride to see what's out there. Couple days ago I spent 8 hours in my Can-am Defender. 46.4 miles for the day...and hundreds of pictures.
As far as whether you need a rear window...ever drive an SUV or something similar down a dusty dirt road? The rear of the vehicle gets covered in dust because there's a suction created at the back of the vehicle when it is moving. If you don't have a rear window in the side by side all that dust will be sucked into the cab area.
But if you are on a group ride that doesn't matter anyway because unless you are the lead machine you will be dusted out by everyone ahead of you. I see people wearing all sorts of dust masks and goggles - even helmets with air filtering systems - so they are not breathing dirt.
Group ride:
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