Posted in the other thread. Microsoft choose to not bother with mobile broadband due to a few factors from what I have read. Most tablets purchased are WiFi only and of the cellular enabled devices only some of them are ever used on their cellular connection. Then there is the issue of the various radios that need to be supported to cover the various needs around the world.
Out here in Nigeria, iPad and Samsung Net Book is preferred, and even enjoying agressive marketing by Telecom Providers all because of the SIM inside. The Interconnectivity with PC Microsoft App and USB is a plus for surface but no SIM is a minus. Might work in the US, But not in Nigeria, the biggest Market in Africa. Wifi is not readily not available and fast internet connectivity is primarily provided by Telecom Operators - Etisalat, Airtel and so on. Nokia Lumia - Windows App is booming here, they should rethink and add version of surface that has SIM to maintain a lead else hp ELITEPAD will take it.
Like Sin said, I posted in the other thread too. The reality is no matter how important it is to you the demand just isn't there. The largest tablet market is by far the US so they are going to make devices targeted at the largest market first. Statistically 9 out of 10 tablets sold are wifi only. Of those sold with mobile broadband only a small fraction of users ever activate it. There are several ways to get mobile broadband on devices without sim cards so there is no loss of functionality (maybe convenience) and no point in making hardware with the option just for the very few people who will ever actually use the feature.