The entire idea of a Speedster was not Ferry's ideal to start with. Porsche was already making Cabriolets, and another drop-top was not in the original plans. They were already making race-cars (550s) when Max Hoffmann asked for a stripped down sportscar, presumably to better compete with the British on price. Max got his car and Porsche got a toehold in the USA. A happy happenstance of the result was a car that came to be a winning fixture in the less rarified air of sportscar racing in North America.
It always required a lot of sacrifice to live with, which is why, with the celebrity set, bad-boy hardcases like James Dean and Steve McQueen gravitated toward them. It wasn't really Cary Grant's jam.
The Speedster, D, and Roadster were different from the Cabriolets, not just in the windshield, but behind the seats. A 356 Cabriolet looks like a coupe with the top chopped off, very much like the 911 Cabriolet. The "removable windshield" cars have a flatter area behind the top, for a sleeker profile. The bodies are quite different from the front fenders back.
Max Hoffmann wanted a stripped down quasi-racer, but what Porsche believed buyers really wanted was the appearance of one, so the '59 Convertible D and the B-bodied roadsters worked to split the difference. "Removable" windshields that were never actually removed, windup windows (like civilized people in cold climates have), and nicer seats were the compromise.
We're not that much different than people 65 years ago, and I've certainly been guilty of larding up the lightweight. I'm trying to resist the urge. There are some features I want to have, and some that just add complexity. I'm never sure where that line is, I only know when I go over it (and I have).
For me (at least) a cabriolet is a couple of bridges too far.