A workspace can not be freed from memory while it has a spatial cache containing references to features.
The spatial cache is also commonly called an edit cache.
The spatial cache contains features that have references to their workspace. The workspace can not be released while there are references to it.
Prior to 9.0, the spatial cache was only in the scope of an edit session and it was automatically cleared when stop editing was called on the workspace. However at 9.0, the spatial cache is maintained outside an edit session and the responsibility to clear the cache has been shifted to the user.