Marco Arment made some excellent comments as to why Apple is not including any sort of cellular data connectivity in any of the MacBook lines. This is one of the only major things lacking from Apple's laptop lines.
With LTE, you can burn through a 5 GB data cap in an hour if you’re downloading big video files, and it would be easy to burn through the cap in just a few days if you’re streaming HD video — which, in 2013, is commonplace.
Back in January, I had some major problems with my Verizon FiOS business connection where only the Verizon-provided ActionTec router would work. The rest of the details are for another post but I ended up using my Verizon 4G LTE USB data card as a stopgap until Verizon could do a truck roll. I only had the 4G LTE USB data card and router as our connection for about 36 hours and many of those hours were while we were sleeping. Using YouTube and a little bit of Netflix from our Apple TV units during that time we easily burned through 8GB of data. I had to upgrade our Share Everything plan with another 2GB so we didn't go over that month.
I have been using 3G EV-DO since around 2005 just like Marco. The advent of 4G LTE has really transformed the way I use cellular data. I would absolutely love to have it built-in to a MacBook of any sort. As it is, I currently either have some sort of router in the car, occasionally use a MiFi device, or tether (actually using Personal Hotspot via WiFi for the most part) to one of our 4G LTE iPads. Marco also makes some good suggestions about what could be done.
To start, Apple could just put cellular-connection detection and responsible-usage logic into iTunes and Software Update. That would be sufficient to launch with new 4G MacBook models at WWDC, then they could have a session on the new API and start enforcing responsible practices in the Mac App Store. Along with maybe working something out with Netflix, they’ll have addressed the biggest accidental bandwidth hogs that most people will face.
It would be very disturbing to have the latest HD TV show episode, which easily tops 1.5GB, download in the background. Unlike with 3G, you might not even notice because 4G LTE is so fast. As Marco points out, it would also be essential to disable automatic software update downloading. As a technical professional, I would be willing to deal with these issues manually but most consumers would not want to be bothered and would not have any idea there would even be an issue until they started receiving data usage alerts or, even worse, a massive cell phone bill.
Built-in cellular connectivity in laptops is also mostly aimed at business users except in the case of the various Chromebook models. The Chromebook is unique in that everything is web-based and not very much data is transferred except for streaming media. Apple may not be including cellular data because of the focus on consumers rather than business users. In any case, I think Apple needs to create a new API for OS X as Marco suggested sooner rather than later. Apple can obviously implement 4G LTE cellular data just fine in the iPhone and iPad lines so I hope the MacBook lines are the next frontier. The next generation MacBook Air seems like the perfect candidate to me.