It is at least the second time of the week when I try to connect while the server is full, considering that I am not as active as I was before. I really like EmpireWar and wish it all good! I hope I can play on it again soon... (requests 32 slots)
We want the server to be full at least once 6 days a week, before we upgrade.
As Tobberz said, a full server increases demand, which is good for popularity, spreads out the playerbase more so it's not just full on weekends at peak time, and makes people donate for slots.
If we notice we are full a lot (and I mean a lot, not just an hour during the weekend), we'll upgrade.