1) because I don't really know what are supposed to be xblock children... I thought they are just not working for the moment and haven't played with any yet 2) we keep saying that LightChildren will be removed, so not trying to base something on 3) I didn't see any benefit to add too much complexity only for that feature, especially when we try to merge it for today 4) we are not that far from the *automatic* xml content loading using XBlock. when everything about xblock child and xml will be working, we can easily modify that part. 

Of course, you know better the soft than I.... so I'm probably all wrong :)