Once issue 196075 fixed, some places in IDE have to know a OS specific placement of root of default userdirs.
* autoupdate.ui, installer - needs it for reading/storing .superId property
* o.n.upgrader, pluginimporter, options.api - needs it for searching older version of IDE (btw. these module still reads system's property 'user.home' for former versions)
Would be nice to use Places API for this in a similar way as for userdir.
You know most about this; submit a patch with usages for review.
Waiting for patch.