Tennis is often played in doors, but most of the big tournaments and all the Grand Slams are outdoors. I' sure it does effect people, and of course rain can play havoc, but that's part of the game. Furthermore every match in the tournaments runs the same risks.
When I grew up, we played basketball in the winter, outdoors, on bitumen courts. You played in the wind, sun, rain, whatever, but again everyone had the same risks.
If its really going to attract big crowds, maybe ok, but what's the attraction?
It's not like you can throw a picnic rug on the hill and spreadout, its still the same venue, just no roof.