I my mind, the main use case for a DelegatedX is where I want an X, but want to override/add in just a couple places.

e.g. _WrappedMap in my latest Multimap CL. I want a Map, but one where operator []= throws. I generally like the idea of making classes mixin-able, but I'm having trouble imagining a scenario where using a DelegatedList mixin would have any advantage over a ListMixin - an example would help.