It's very unlike Apple to deliberately ignore an opportunity to sell a bit of convenience as a premium. Lots of ipad owners own iphones and yet buy the ipad with a 4G modem that they technically don't need. An always on Mac Book Air that "just works" is such an obvious thing to do given the target demographic of people using this on the go, that there must be another reason than "we already sell iphones/ipads". I don't buy the "users don't need this argument" since, I see way too many people thethering their macs with separate access points that are definitely not their phone for this not to be a thing at least.