It's probably the same as with Pokemon Go[0]. The game is fun on its own, and the geo aspect makes it different and get's you out of the house. But the reality of the VR is grossly overrated and more often a nuisance than something adding to the experience.
Pokemon Go gets better when you turn off the AR mode.
Being able to virtually build something in AR mode in Minecraft seems pretty sweet, especially if your friends can see it. Getting the AR to work right won't be easy though, phone positioning is imprecise and it will be really hard for the phone to line something up exactly the same if your friend is viewing it from a different angle.
This is one of those game concepts that looks like it would be fun an easy but turns out to be a real nightmare to implement I think.
[0] https://www.youtube.com/watch?v=2sj2iQyBTQs