MHP can be extended to support 3D using any of the existing Java specifications for 3D. The one which I've heard the most interest in is JSR 239 - Java Binding for the OpenGL ES.
Being open brings many commercial advantages. If a manufacturer or operator wants an MHP implementation, they can go to Osmosys, Alticast, NDS, IDWAY, Motorola and probably others as well. If they want an OpenTV implementation, they can only go to OpenTV. With multiple different suppliers, a manufacturer / operator may have wider choice of processor and operating system instead of whatever a single supplier supports. The same is true for broadcast tools. Where a single company owns the middleware specification, often the broadcaster tools are only available from them.
The MHP vendors and proprietary middleware vendors compete for business with network operators. I would expect similar features.