It depends what you want to do in academia. The combined degree programs are primarily for people who only want to do academia and IN ADDITION want to do research (hence the PhD). If, for example,you just want to teach in the clinics, you definitely don't need a PhD for that. You can actually have your own practice and teach part time or you can teach full time. I'm not sure of the exact path that most clinical professors take but I don't think you need to be certified to teach. I think having the DDS/DMD is enough. Not sure on this though. If you are interested in research, you don't need a PhD but it's HIGHLY recommended. You can still do research and be very good at it, but unfortunately, many are biased against researchers without PhDs. If you are at a dental school as a full, tenured professor, and you do research, you're pretty much required to teach. The other thing is that PhDs aren't taught to teach, it's just expected. This is why you get a hodgepodge of professors: some are really good at the teaching portion and love it, others hate it and do it only because they have to.
Adding the PhD takes longer but you get paid to go to dental school (most programs offer tuition, fees, health insurance plus $20,000 stipend). The assumption is that afterwards you will do a postdoc (with or without a specialty) and then take a faculty position at a dental school. This is preferred, especially with the growing shortage of dental faculty members, BUT you certainly won't be forced into it. There are a few cases where the students just go into clinics instead.
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