I'm late to this party. We have six kids (BGBBBG). We have *always* had our kids share a room, until last August when our oldest daughter left for college. Until about two and a half years ago, all four of our boys shared a bedroom (~12'x14') in two sets of bunk beds. At that point, it was just a bit too crowded with four boys aged 10-17, so we split them into two bedrooms. The girls have always shared ever since our youngest was born. Our youngest (10y.o. girl) is now sad that she has nobody to share a room with. When our oldest son left the house a year and a half ago, DS2 assumed that he, as the now-oldest son at home and only high-school-aged boy, would keep that room to himself. He was bitterly (and I mean full-on teenage "you're ruining my life by not giving me my own room" angsty bitterness) disappointed when we moved him back with his two younger brothers. DW and I have held firm, however, despite his many complaints and arguments.
Why do we do it?
1) Avoid instilling a sense of entitlement. At times, this particular son struggles to appreciate how nice his life is, and making it even cushier is NOT in our program.
2) Learning to act appropriately around and with others. This means learning to appropriately handle it when your younger brothers forget to put their dirty laundry in the basket, how to go to bed and get up without waking your roommates, how to sleep when there are other people in the room, how to share space.
3) It fosters relationships between the kids. They're "not supposed to" talk after lights out, but they do, and we only half-heartedly tell them to knock it off and go to sleep.