If Apple ever release Surface like device, I will buy with heart beat
You see MacBook Air + iPad will indeed give you better experiences. Apple really perfected each devices for its intented purpose. There is no question about it. However, with mobile phone comes alone, we no longer need to carry an MP3, Camera, a mobile phone with us. Everything is so compact and easy to carry? However, do iPhone provides better for each experience each individual functionalities? No. iPhone's camera is no where hear DSL. Sound quality is no where near good music player, iPhone's battery is no where near a dumb phone, however, iPhone indeed provide a better experience when you integrated ea of these thing together, even though it may not perform better than each individual product. This suboptimal experience provides us convience.
Have you ever need to carry iPhone, iPad and MacBook(Air) with you. I do and lots of time. For example, when you need type some report, you need MS office. The iOS version of office provides nearly fraction of desktop version offering. No double space, no formatting etc. So when you do need bring multiple devices at once, you will feel Surface type of devices really comes handy. I am willing tread better experience with conviences.
So if Apple ever put Surface type of devices on the market, I would buy without thinking. It will just go buy. However, there is no need for me to buy iPad and MacBook, because even iPad mini first gen is enough for the intended purpose.