The new methods are there to support LINQ queries. They are mentioned in
the 1.4 release notes:
If you want to write LINQ queries against values serialized by your custom
serializers you should implement the new methods. Otherwise you can add
stubbed implementations that throw NotImplementedException or
Consider deriving your custom serializers from BsonBaseSerializer, which
provides default implementations of the new methods.
These interfaces are changing yet again in 1.5 (sorry).