Just yesterday I was bitching about problems with my Spyder, so this defense of the car may seem schizophrenic. Additionally, the following is just my humble opinion...I don't want to confuse the issue with facts.
From a purely aesthetic point of view, the attractiveness of the 550 replicas is their relatively authentic nature. The original cars had no amenities to speak of and the replicas are designed "replicate" that experience. So the way I see it, Spyder owners have the choice of risking getting wet driving in the rain OR leaving the car under the cover and driving the daily driver...And this is what I sense that most Spyder owners, except the mildly masochistic Vicki V. and a precious few others, do.
From a production/cost point of view, adding roll-up windows and the requisite mechanical do-dads, seals, etc. would only serve to significantly increase the build complications and costs, both of which would be passed on to buyers in the form of an increased sell price.
Personally, I when it comes to weather, I regard my car the same way I would a motorcycle...The increased probability of rain/cold increase my chances of getting cool/cold and/or damp/wet depending on how cold it is and/or hard it rains.
And without at all intending to be a smartass, people who want a completely climate controlled sports car have numerous options that fall within the price range of a new Spyder...In fact, this month's issue of Grassroots Motorsports has an excellent article on used Boxsters, Audi TTs and BMW M-3s...And don't forget that a new Miata and Mini can usually be bought for less than a new Spyder.
Again...Just my humble opinion.