If you haven't used a pinlock shield before, just buy one. The pinlock is a second lens that seals against your shield, trapping a layer of insulating air between them. It makes it virtually impossible to fog up when you're riding. I have been stuck in traffic, in the rain, temperature in the 30's with my shield down and it absolutely will not fog up.
At night there are sometimes weird double-reflections of lights but this doesn't take long to get used to, and it's still better than having to ride with your shield cracked open.
If you change out the pinlock lenses alot the seal can wear out, I recommend going with the clear insert and just never removing it. Once you've got it installed with a good seal dirt can never get between it and your shield so there should be no reason to take it off.
The little pins in the shield have an offset cam shape, rotate them to change how tightly the insert is held onto the shield.