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In my mind, the solid surface gives the air something to push on as it falls back behind the vehicle, this push, in addition to the lack of vortex generation derived by a properly shaped form is what gives us a lower drag coefficient. Not sure how well the mesh will allow the air to push. Not saying this will give you worse mileage, I'm pretty certain a mesh form will not give you anything close to optimal for a given shape. If in your mind you see 95% of the air "Skipping" across the mesh, I see 50-60% pushing straight through it. One of those pressure gradient things where the air wants to go into the low pressure area and does not care how it does it.
I've driven behind a new beetle in a rainstorm before and seen the huge counter rotating vortex's generated by the serious down wash created by the air moving over the top of the rear window and being slammed down. It is quite impressive. I'm pretty sure this is where a large amount of drag is created and why a large "Whale Tail" style spoiler is effective in reducing drag by disrupting the formation of these large vortices.
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