The Car Talk answer was that the increase in drag is worse than the drag of the AC.  

I recall a funny comment by a Daimler engineer saying that they put in sunroofs so you wouldn't need AC, but Americans insisted on both.

Opening the windows is not the only option!  All cars have vents, with fans.  That seems to do the job for me, no need for AC.  (But I live in Vermont...)  Opening windows makes a lot of noise.  And those "sunroofs", I've never figured out what they are good for.  They make a lot of noise, little air flow, and let the hot sun hit your head.  ??!!
"I've never figured out what they are good for." Use your imagination a little -- they make great mobile guillotines. Come the revolution... .
Just joking, Dick, Rummy, just joking. Hm. Shouldn't be giving these guys ideas, should I?
Much better is to have front-seat passenger stand, armed with assault rifle or light machine gun resting on metal part of roof, fully loaded and at the ready to take out discourteous drivers with tracer bullets to the fuel tank;-)

We really have to do something about road rage.

I live in Dallas and am a hot-weather person. I normally drive around here during most of the summer with the vents on natural air and windows rolled up. Only when I'm in dress clothes (normal work outfit is polo shirt and khakis) do I turn on the A/C.
When I went to college there, we'd drive around with the windows up and the heat on looking for Yankee hitchhikers.
The manual sunroof on my Accent has an internal panel to block the sun, but can still be poped up at the back.  It vents very well that way, without allowing the sun to beat on your head.  Also, I always park it that was in the Summer, as it keeps the interior MUCH cooler.  In the winter I keep the panel out to let in more sunlight.

I often see people get into a closed up, boiling hot car and turn on the AC without opening the windows at all.  This puts a huge load on the AC.  If they would just run a block or two with the windows open, it would make a lot of difference.

Sometimes people would run around in hot dry places with wet towels draped over partially open car windows.  I think this was supposed to be an improvised "swamp cooler."  I think that a sunroof would offer great potential for this, with a continuously wetted loosely woven towel attached underneath the sun roof.
They're damn good for getting rid of cigarette butts and beer cans.
I once had a car with a sunroof. I called it the turret. Thinking of Road Warrior, cars with sunroofs would be great as "technicals" as a gunner could man the turret in a road battle for the refinery tribe's oil.

In Iraq, gunners man turrets that are basically sunroofs in the Hummers. By armoring a car with Plexiglas , the turret is invaluable in road warfare. (Think of The Gauntlet with Clint Eastwood armoring the bus cockpit but replace steel with Plexi.)

The main usefulness is if a Road Warrior -ish future develops.

Up to low to mid-80s (F) outside temperature, I open the sunroof on my 1982 Mercedes Benz 240D.  I have "premium" reflective film put on the car to reduce heat loading. And it is white, the one color that significantly reduces heat gain.  And I keep it waxed with a nice reflective wax (Turtle Platinum).

I also use LEDs for marker lights and am considering HID conversion for headlights (brighter, 35 watts instead of 55 watts on low for each headlight).

Synthetic fluids all round.  Mobil 1 in engine & differential, M-B in manual transmission, Valvoline in Power Steering and brakes.

Result 31 mpg city, 35 to 42 mpg highway (depending upon speed) and a car that will last my lifetime, barring accidents.

Our 1983 240D remains our favorite car - "under-powered is howthe mechanic described it; but we loved it with manual transmission, sunroof and pretty good mileage.  As a confirmed claustophobe, I like a sun roof, and find that it vents the car well and does provide fresh air, especially for the back seat. Alas after ~380K, it sought late retirment in a local auto retirment community