There have been changes in registry settings regarding tool bar settings between some of our versions. Version 3.41 and 4.0 are change points of this kind.
If you have started a 3.41 or later without this switch you are not able to start an earlier version without deleting the tool bar settings in registry. The same goes for the version 4.0
If you want to run a 3.40 together with a higher version all the higher versions must have this switch. The same goes for version lower than 4.0 if you run the version 4.0.