Sprint compatibility is not going to happen, It is Verizon and T-Mobile only. If it can work with later model Verizon iPads which can be also work with GSM LTE carriers, it should also be possible for something similar to be done with a Surface, The tablets are LTE data only, so they don't have as many compatibility issues as a CDMA voice phone,
Earlier Verizon iPads only worked on Verizon, but after Verizon switched to LTE data and Apple updated the iPad hardware,, compatibility became better.
Most people will prefer to tether to avoid a separate data plan bill, but LTE is needed for everyone else.
Sometimes you don't want to burn your phone battery charge out by tethering or else the data plan is being paid by your company so tethering from your personal phone is not an option. Built-in LTE is more convenient that either using a MiFi or tethering to a phone.