It isn't just jeeps that were able to have he windshields fold down.
My 2 1/2 ton 6x6, made by AM General, does this as well.
1971, truck, cargo, 2 1/2 ton, 6x6 M35A2

 

The theory I always heard was so they could shoot from the front seats out the front of the vehicle.

 

that was my other theory also

 

Well, I thought the theory was that they would get everyone used to doing it and it looking cool and being easy, then when they got to the JK model they would make it really hard and not look as cool so they could sit back and laugh at what they had done.

 

I found that when driving on a dusty road with the windshield up (CJ-5) I got a lot of dust in my face that swirled around from behind the jeep and got sucked into the vortex behind the windshield. With the glass down there is no vortex and no dust in the eyes. I think many of the above listed reasons are likely though. Makes it much easier to shoot from the passengers seat!
I agree that the JK looks goofy as hell with the windshield down, not right at all.

 

i heard it leaks after you take it off

looks weird with it down too