I have run my car with and without the wind deflectors and in my opinion, the only useful purpose they serve is to keep the mirrors clear in wet weather. When fitted, they also appear to increase the amount that the sidescreens are sucked outwards when driving into headwinds. Currently, I have removed them and I think that it will be unlkely that they are refitted, so my advice would be don't bother.
I tend to fit a set for the summer, when I usually run without the door tops, and remove them for the winter, when I usually have the door tops on. It's 4 screws and less than 5 minute job. I'd be the first to concede that they don't do a great deal, but IMHO they look really nice, so for 5 minutes twice a year I reckon it's worth it.
