How do you know that? If it can, someone likely will. Enclosed means it's out of the weather too. Being open topped means, you get to enjoy the heat, cold, rain, and whatever to the fullest, along with potentially a sensitive cargo subject to damage because of it. Convertible cars are the exception, not the rule. Open bed trucks are the exception, not the rule.
An 'air raft' can go to orbit. You can bet if it can, people will. Adding a cab or enclosed passenger space, and even enclosing the cargo space won't add much in weight (yea, you can all technical and say "mass"). It would also add streamlining making the vehicle more aerodynamic (less drag) than an open vehicle.
In terms of a Traveller game, if the party (players) were doing something involving combat I'd think depending on the scenario they might well want to be using it at a higher altitude to avoid ground fire as opposed to being NOE or at low altitude. Think something like a Blackhawk Down scenario of some sort. Flying low over an urban area gets you shot down, while flying higher avoids ground fire. And, at the same time, having an enclosed passenger compartment gives you some protection from fire either way.