The reason why moulded boots or blades are not allowed in rugby is because they could potentially cause harm to other players and the person who is wearing the boots. Moulded boots don't have the same amount of turn as a normal rugby boot and can cause injuries such as broken ankles and muscle strains which i have experienced myself. Also steel blades could be sharpened not maliciously but could cause cut, this is why referees check studs before kick off.
I hope this helps
Jack