I just ran into a bug with this, I believe. I have a Blog which cascade deletes it's Feeds. In cascadeDetach it's saying the count($relatedEntities) is zero for Feeds after the call to unwrap(), even though I know it to be, say, 1 or 2 (output before detaching owner entity). However, if I do ->count() or count($relatedEntities) BEFORE unwrap(), then PersistentCollection by consequence is initialized, and it cascade deletes correctly. If it don't, it actually says the Feeds are still MANAGED after having detached the owner entity. So, yeah, this or some other form of auto-initialization would be useful.
Note: In Blog, I have DQL which fetches Feeds JOINed to Blog, which may be causing the uninitialization (not sure). I'm using EXTRA LAZY LOADING.