I've always believed that a mobile hot spot is the best solution when it comes to connecting non-smartphone portable devices to the internet. Your device is not stuck to one carrier and prices are cheaper. Plus, hot-spots aren't that expensive without contract so you can just activate the service in the months you'll really need it. The only benefit about built in data is that you spend almost no time connecting. That's great, but IMO the benefits outweigh the cost.
If you tether ALL THE TIME (like I used to) it's convenient and useful. It tends to be a bit faster (especially with modern hotspot boxes focused on being small and sleek despite performance disadvantages) because it can hide larger antennas in the display, it doesn't require another device to be setup, etc.
But, that's why it's a niche product not likely to make it into the MBP line. Years ago, when I used to tether almost every day, I'd have kiled for a 3G MacBook Pro! Today though, despite cell speeds being faster, I tether less, a mobile hotspot or just my iPhone works just fine.
I also use my iPad as a Wi-Fi hotspot (jailbroken), the beefier iPad battery means you can get several hours using it as a hotspot device. It's a 3G-only iPad but that's not a big deal in an area where that's about all you get anyway. When a jailbreak becomes available for LTE iPads that can be a great solution too. (I'm always a fan of using the gadgets I already have and am using instead of adding more!)