The MGB I had with a hard top and it didn't need a roll cage for the hard top. I would think that the BMW is probably more rigid than an old MGB.
Would it matter if there was slight movement at the windscreen join? It would have a rubber strip to allow for movement and weatherproofing.
I can see that a bonded on hard top would need the roll cage to stop any movement and prevent cracks forming.
The roof itself could have a couple of supports glassed in, in the same way as the bonnet has.
My 250 convertible rear end is only bolted on with the bolts at the front of the rear wing in the door apertures. It is meant to be fixed at the bottom of the rear wing with a bolt on either side but to fit them I would have to some strain on the rear clam so have never fixed it here.
It seems to be on very firmly but I'm sure it gives it a little room for movement which hopefully will prevent any stress crack from appearing.
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