In Revit 2016 R2 wall joins received some long overdue love from the programmers.
There is an interesting thing to consider when placing walls. Look at the following
I have create these few walls. The only difference was the creation settings for wall joins. The top were set to allow. The bottom set for disallow.
The interesting part comes when you start to use the join and unjoin tool.
The unjoin tool has no effect up on the walls that were create with the setting for allow wall joins.
Walls that were created with the disallow option can be joined and unjoined. You may keep doing that as many times you need.