The primary advantage of MDF is dimensional stability. Dimensional stability is highly desirable for surfaces receiving veneers or high pressure ("plastic") laminate finishes. It's not really intended for structural applications, but for furniture and cabinetry it can be a reasonable option.
Another advantage MDF has is that it's heavy. Granted for most things you'd rather have light, but for stuff like speaker cabinets and sound deadening you want heavy.
Also, MDF makes a bad material for speaker cabinets. A good speaker cabinet not only is heavy, it have a really good characterized sound qualities that MDF, plastic or cheap plywood can't give. Because that are expensive!
