That's why said optionally on the motion control.Thanks for the idea. I have played with mobile minecraft's controls. I don't really like them though. Granted I'm not gonna get as good controls as the PC, but I have a few ideas. As far as motion controls those would be non essential controls likely. Since its hard to make sure you are in a correct orientation on a bus/in bed/other places you might try to quick wipe out a ship shape.
You could use the screen as a pen pad. but then you are left with running a reverse line back to determine where the pointer is hitting the target.
While that works on a PC the touch accuracy on most mobile devices isn't near that accuracy of a mouse or even my pentab for drawing.
So you will end up with the placement box bouncing around.
Which is why I said go with the minecraft controls. When it comes to mobile devices they have a lot of quality difference not to mention difference in size.
Honestly, look at it this way. The people building minecraft had to look at the same issues. They didn't just slap a set of controls on and go we are done. The problems they ran into will be the for the most part the same you will.