Why can't you set your backboards plumb and square using shimming or something else instead of floating walls when installing tiles?
You can shim backer board.
Depending on the tile size and how it meets with a seam I will mesh tape and float my seams to reduce cracking.
If a tile seam ends up close to a backer seam. This is where I float a wall not to try and square things up.
A vertical surface being filled with mortar to square it up just sounds nuts to me.