Our theatre room is the kids playroom. If we watch tv (which is very rarely), we watch in the family room. We built and shuffled the plan around a little and for us right now with 2 littlies, its all we need.
We plan to build again once the kids are older and will obviously have/need more space as they will need their own room too. If its your 'forever' house then yes, I'd be thinking more to the future than now