Dammit, that's exactly how I was hoping to use it, MacBook in clamshell using AirPlay to the iMac with the iMacs input. Of course, that wouldn't be entirely necessary if Apple would just make AirPlay scaling work properly. Or let us use the iMac as a monitor without signing in! God I miss Target Display Mode. That's all I need, to be able to bring my MacBook into the office and use one of the hotdesk iMacs as an external monitor. But yes Apple, I get it, you want us to do everything through iCloud so you can try generate yet more income...