I think people under the same roof should all be able to stream no matter what kind of Netflix account they have. Being able to determine who is in the same house and who isn't is a whole other can of worms - do they base it off IP? Registered wireless devices? Have an arbitrary limit of devices you can connect and register?

This is seldom an issue for us personally, as we have one TV in the house and none of us prefer to watch movies sitting at our computer. On occasion my husband has business trips out of town and he'll watch movies on his laptop. We've never had an issue, though we haven't tried streaming from two devices simultaneously recently.