Why would you not need mobile internet in a LAPTOP? Isn't the whole point of laptop to have internet when you're not home?
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They could put the antenna in the plastic hinge under display maybe?
Look, they now have the iPhone and the iPad with internet ability, why create another product with the same? And I do not need the internet the whole time, I actually cherish the times when I do not have it while out and being about, even when I have my MBP with me.
And other notebook manufacturers also do not offer that as a widespread option (maybe there are some models having that capability, but they are not widespread as any MacBook or MacBook Pro or MacBook Air is).
There are enough third party solutions like tethering or dongles to connect a notebook to the internet. It is the same as with the Apple Remote or a Mini DVI to DVI or VGA adapter, they initially came with many new Intel Macs back in the day, but as Apple realised, that not many need them, they sold that separately. The same probably goes for such modems. And do not think, just because you or others you are acquainted with might need them, that others do.
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Well... now you are going a bit overbroad with the minimalism

If such a feature does not result in cost increase and does not sacrifice anything else (such as the dimension of the laptop), it would improve the usability of the MBP tredemdeously. Although its true that many places have WiFi... for instance, in Zurich I can find a WiFi almost everywhere (and as a university employee I can use all WiFi points for free

)
That was the next thing I wanted to add but forgot, the widespread availability of WiFi, at least in cities.
But on the other hand, by brother lives in the country and could need such built in modem, as there is no landline where he lives, bit moderate HDSPA availability.