As far as I know, DJ, only the P39 has the glass that reflects light inside the cockpit in Il-2.
I've never noticed it with the other planes.
There really are no benefits to it, however, you will be unable to see any plane opposite where the sun is glaring off your cockpit glass.
But, of course HTC would have to make it so the sun doesn't shine through your plane too... as it is now, if the sun is below the terrain (night time), the sun glare effect on your dashboard and other parts of your plane's cockpit are still there. As if the sun is in your cockpit.
-SW