I've used two Noah covers on my two Jag sedans for a couple of years now, in England and here in So Cal.
My personal reasons for keeping a cover handy is to prevent water, dust, and sap from sitting on the finish. Also, it keeps the temp down and protects vs UV if your in a sunny climate.
The Noah is a great car cover but for anything more then a light rain, you will have a thin layer of moisture underneath with some drops in a few aeras. You will still need to either drive that off or use a chamois to pick up the residual moisture. If your in a rainly climate, there is a fabric called Supremeweave touted to be better then Noah with better environmental protection.
You can flick it and water comes right off rather then staying trapped in the fabric. What little moisture that does remain but can't be wrung from it after a heavy rain will not mold. England's weather and a rain and then sun in So Cal have not molded either of mine a bit.
The Noah makes for a fairly decent dust cover for long term garage storage, but a dust cover is much better suited to this.
It is thick and will take up that 1/3 of your trunk space on an NSX.
The Noah is pleasantly thick and will offer protection for people who may brush against it and offer protection vs doors that "pop" out to lock at the end of their pivot and are close enough to ding.
Its machine washable in very very light detergent.
www.calcarcover.com (No affiliation) will custom make a cover to your exact dimensions. A big advantage to this company is it makes its Noah from sewn panels. If a cat decides to go to town on a particular part of your cover, that panel can be replaced for less then $10-20 including install without having to buy a whole new cover.
Supremeweave would be the way to go. Had it been out when I bought mine, thats what I would be using today. I paid $260 or so for my Noahs to cover a sedan 198in. long, 71in. wide, and 53in. tall.
Those rules posted early on when/when not to cover are something to consider. If its a light dust, use a duster. Anything else, don't cover it until you wash it. If your worried about light scratches from the cover or a duster, why are you driving around without multiple layers of Zaino or a carnuba like PS1 or Pinnacle anyway??
Sunny