Many research advancements involve interdisciplinary research. However it can be difficult to achieve in a doctorate. Some institutions only recognise doctorates in one traditional subject area even if the thesis is interdisciplinary. Perhaps the interdisciplinary area needs to first become recognised as a new discipline in its own right. I had this problem with my doctorate.
It definitely identifies a researcher as a specialist. When it comes to a particular field, would an interdisciplinary PhD qualify a researcher as a specialist in a particular field?
If it is still useful, I can add that I have an interdisciplinary degree in education. In my experience this has meant I can focus more on my overlapping interests yet have had to also argue more succinctly and deeply in some situations (eg jobs, funding). While interdisciplinary research is frequently encouraged now, academia still frequently works in very clearly designated fields. You can become a specialist then as an interdisciplinary scientist but will just want to also see where you fit within the broader topics as well.