I used to drive with mine like this, either if it was raining lightly, or sometimes in the summer if it was very hot & some shade was required. It worked fine for me, but I never drove particularly fast with that set up.
Yes, I have also driven like this on occasion, but I am concerned about the stress on the windscreen and pillars, at anything over about 40 mph there's an awful lot of fluttering/thrumming of the roof, which is bound to be transmitted down through the windscreen.
