Unfortunately, there is a real subset of users who can't just use their phone. From my own usage case:
1. I can't talk on my phone and use LTE simultaneously.
2. The surface can't update in the background by itself.
3. LTE from my phone drains the battery quickly.
These issues may not seem like much to some people, but to those of us who are on the road constantly, the lack of LTE is a significant impact to our productivity.