I have a 4G Chromebook and I rarely use the mobile data. However, rarely is not "never." The 200MB of free T-Mobile data has come in handy several times.
200MB is enough to send a receive email and do a little web surfing when away from WiFi,
If the Surface 3 qualifies for the same 200MB free data monthly as other tablets, it will be nice to have depending on the extra charge. I probably won't get it because I would go for the Costco bundle for personal use and the is no LTE version available for pre-order through Costco.
If we were to get Surface 3s for employees, LTE versions would be really nice for a subset of frequently travelling employees so they don't have to carry a MiFi portable hotspot around with them that is another device to charge, forget or get lost.
If we are able to pop out the SIM from the MiFis we already own and pop it in a Surface 3, using the same data plans and accounts we already have, it would be a nice convenience,
Those who need to share the connection with multiple people/devices would be better off carrying a MiFi though. Even though it is possible to share an Internet connection via Windows ICS, I think the concept and more simple steps of turning on a MiFi hotspot will be easier for most users to understand than getting ICS working reliably and securely,