This is a huge factor. When most people are in the office, the means of communication becomes different, in tons of little ways.
For example, brainstorming sessions end with scribbles on whiteboards, and a fleet of Post-Its that say "Do Not Erase!" It's perfectly adequate documentation because the same team comes in the following afternoon and keeps going. But the one or two people that work remotely are never going to see it. And no matter how much Skype you do, it's just not the same as being in the room as it happens.
For example, brainstorming sessions end with scribbles on whiteboards, and a fleet of Post-Its that say "Do Not Erase!" It's perfectly adequate documentation because the same team comes in the following afternoon and keeps going. But the one or two people that work remotely are never going to see it. And no matter how much Skype you do, it's just not the same as being in the room as it happens.