Quoting from JDK-8180794:

"The -o option of the idea.sh shell script is almost impossible use and should be renamed (since it is used only for testing). First, the operand is relative to the top of the repo. containing the current directory, so it must almost certainly be a simple pathname. Second, it must have a final path component "idea." if the IDE is to recognize it. These factors mean you can only say "idea.sh -o .idea" to get meaningful results. A real -o flag would place the result relative to the script caller's current directory, and would ensure that the final result had the needed ".idea" component.

Given those improvements to the -o flag it might be possible to set up several IDE projects in one repo. This would be helpful to some programmers especially in repos with many branches, although others will continue to make one repo per active branch. "