Mainly a personal choice. Some prefer to drive with the roof and windows retracted, so a rear windscreen/blocker is of no function in this situation.
Some prefer a bit of flexibility to drive with the roof retracted and main windows up, so the rear windscreen cuts down on turbulence and helps in keeping the cabin at a comfortable temperature during winter months.
I have been driving in 4*C weather and I find that it does help to keep the cabin warmer than without it. I have a factory MB net but I'm really looking at getting the Windrestrictor model. Does anyone know if there is a difference between both net and plexiglass or just the look?