Use of materials is greatly influenced by the intended design. personally I only really like Die Case as ballast e.g. elements of feet or something as mentioned by Goki.

metal is much harder than plastic so any moving parts where the two meet, and metal is going to win every time wearing the plastic quickly and making a joint loose.

Metal also fatigues a lot faster than plastic, there is no way die cast metal could be used for most applications in modern transformers as a direct substitute for the plastic being used at the moment. it might be stronger at face value, but, I guarantee you it will fatigue and crack much faster. In parts that are thin it will also bend and stay bent, where plastic can yield and then take it's original shape. Die Cast can do this but not to the same degree as most plastics used today.

my 2c