At the day-care my 3 year old goes to, the children are encouraged to go out and play on a daily basis. The only exceptions are storms and extremely hot days (this does not happen that often in this part of Germany...). This is great for them. They play, explore, breathe fresh air, interact with germs, bacteria.
On the other hand, I'm also sympathetic to the parents who try to shelter their kids, so that they don't get sick. Staying at home these days is not as simple as when we were kids. The pressure to succeed or lack of personnel take their toll in such situations.