First, there are no versions of the Surface with a cellular modem. If you use a tablet on the move you know that you need the Internet, without it your tablet is just a great way to play solitaire. You can't consume content without access to the internet You can't update your status without access to the internet.
Second, a keyboard is sold as a recommend accessory. Laptops are for typing, tablets are for checking the news and your social networks. Tablets are for typing short replies with the tablet in your hands not on a table. Even the commercials show the Surface on a table looking like a little laptop. I've never seen a commercial for any other tablet (Android or Apple) where it was being used with a keyboard. Why, because they are not little laptops, they are content consumption and communication devices.
The Surface was designed to be showcase for the functionality of Windows RT, as such I have to assume that the design choices made by Microsoft were made very deliberately. Those choices show that Microsoft is still looking at the world through the model they find most comfortable -- the PC -- at their own peril.