It's important to us. If we lived in a very built-up area, then it wouldn't be so important provided that the kids had some kind of shared outdoor space immediately accessible. Given that we live a 5-10 minute walk from the nearest park, some kind of outdoor space matters...although it wouldn't need to be huge.
My DD (nearly 4) would happily play inside all day every day. I've made a very concerted effort to ensure that our outdoor space is appealing to her. I'm forever adding new loose parts/tools for digging & making things, adding things to give her ideas (eg. putting a branch in the sandpit like a tree; putting out a basket of shells on the grass etc.) Even then, I sometimes have to insist that it's time to play outside, you can come back in when I've finished XYZ. I think there are so many things that children learn and practice when not directly supervised outdoors that are much harder to do inside (aside from the benefits of space and natural light).