I can only assume it'll be cheaper. I suppose you cab compare it to a 3 series and how theres a coupe and a convertible. The grey one in the last pic looks great but I would always choose the convertible
The grey one IS the only one that looks good because it looks most like the origina.
Could only SE a reason for it if the fixed roof meant lighter, faster and better handling
OP's pic reminds me of the Z3 Coupe which is quite a rare beast these days. In fact it was always quite a rare beast. Mainly because it just wasn't that popular so not many sold. They do have quite a cult following these days but whilst I can appreciate quirkiness and rarity I still think they are butt ugly cars.
Ojiako you beat me to it. I was just going to say that all but one respondents on here so far have said how ugly they think the car in Tim's picture is but if I stuck up a picture of a Z3MC it would be swooned over by many. How is that?