XSI scene graph

Note: They don't seem to be on the XSI blog anymore. It's a shame they were excellent.

This is a very good article about XSI scene graph.
http://www.xsi-blog.com/archives/57
Though it looks quite different, basically it's not so much different from Maya's dependency graph, it has connection, lazy evaluation with dirty. But an operator can modify data itself without caching, it should mean it has more freedom of designing caching mechanism when creating a custom operator, though it is more difficult to create one. And having an operator stack in a data object is good, you can see the construction history of a node at one place with the data itself. Letting us free from nasty reference data wrangling is also good.

I'm just wondering why FxTree (what is this? I'm not an XSI user) is a pure operator graph. There's a history about the birth of FxTree http://www.xsi-blog.com/archives/77#more-77 . Because it is usually not a good idea to mix two concepts, I just wonder if it is just a historical reason or it gets some benefits from being a pure operator graph.