Are ribbon/AMT/planar tweeters just eye candy at this point, a must have for boutique brands to justify their prices? What are their redeeming qualities again? I rarely see smooth treble response from them, and directivity is always sub-par. But I guess they bring that "sparkle"? I'm a skeptic, I think they need more advancement. A good dome tweeter in a waveguide for me.
This is an interesting question and to a certain degree I agree with you. The vast majority of AMT/Ribbon/Magnetostat tweeters do show higher distortion at higher cost than a good fabric or metal dome, but they have some interesting qualities and have a place in high performance speakers.
For one, really good ribbons do have an intrinsically flat and smooth axial response with crazy HF extension. Distortion is an issue, but in a 3 way design such as the BMR, it can be mitigated. In general, the RAAL and Viawave tweeters are extremely high end units, but they can be misused.
Another positive trait is that their relatively large vertical and or horizontal dimension actually makes them a better match for a midwoofer in the crossover region, since they will have a narrower dispersion at these low frequencies compared to a small dome. This, coupled with a modest waveguide, is why the AMT units in ADAM monitors perform so well off-axis.
I've actually investigated whether the assymetrical radiation of a ribbon could actually make the assymetrical radiation of an MTM work better than a dome tweeter, essentially creating a speaker with wide horizontal and narrow vertical directivity, but with a smooth overall power response and DI. It's not an easy thing to simulate but I suspect it can be done.
A more niche use of the ribbon is that they can actually play very loud if you make them big enough. There are ribbon tweeters used in sound reinforcement systems, and they are so fundamentally different from compression drivers that I suspect they offer designers a lot of flexibility there.
An even more niche use of the AMT/Magnetostat is that they don't require back chambers to make sound, so they are ideally suited to dipole speakers. John Kreskovsky the dipole guru used B&G units in his early designs.
However - if you see a ribbon or AMT, flush mounted, in an 'audiophile' two way speaker, you are right to be skeptical.