Is the master's degree funded? If it is funded, I'm thinking there isn't a serious downside. If it isn't... well, then there are two downsides. One is that you'll have to pay. The other is that they may admit a whole lot of people to their master's program and relatively few to their PhD, with the obvious eventual complications.
In either case, when the time rolls around again for you to be applying to PhD programs, I would strongly suggest you apply to multiple programs, just in case. Because too many offers never made anyone sad for long.
As far as an MA/MS being a plus or minus to a PhD application: I have no idea. I really think that nobody knows. Or, rather, everybody knows. Lemme explain. While high GPA and GRE scores are unambiguous bonuses for an application (nobody ever said "No! I won't take her! She got a 780Q, and that's just too high, damn it!"), almost everything else is open to interpretation by whoever is doing the admitting. So some professors (or some programs, or both) see an MA as being almost required as a sign of seriousness and ability to do graduate work. Others may see it as an indicator of not being able to get into a PhD program the first time around.
When I talk to faculty, the fact that I practiced as an MA-level therapist for years has been described as: a. "Great!" b. "Inherently unethical, no matter what the APA and/or state says" and c. "Irrelevant." Which is right? All are, of course, since they're all opinions.
I would really like a comprehensive survey of decision makers (and decisions made).
This is, of course, just my opinion.