Ok... Take a jelly-filled donut...
Punch a hole in it (sunroof), still one piece, still solid.
Cut it in half, not connected, not solid atall...
The only thing being compromised with a custom sunroof is the sheetmetal, you can run structural pieces down the side. Whereas every link is compromised with a targa.
Man you know insomnia is really messing you up when you equate car roofs to jelly donuts, but it seemed to make sense when it first popped in my head...
--BillyM