I would suspect that the biggest reason is the limitations that that imposes on the underlying serialization implementation, as it would require that the underlying format support a minimum set of types.
I'm not sure that's actually true. I've been working on my own serialisation library in D that I plan to unleash on the announce forum soon and it does it in a manner described by the original poster. Even with custom serialisations, client code need only define one function for both directions.
The only reason I haven't announced it yet is because I wanted to be pure it's polished enough, but maybe I shouldn't wait.
Atila
