Interesting! I haven't properly tested anything with a ribbon\long tweeter, so I can't speak too much about the effect on the sweet spot. I suppose even if the direct sound isn't too far off, the whole thing gets "shifted" so maybe the overall impact is higher than I assumed. For horns and deep waveguides, it's harder to separate the vertical from the horizontal narrowness.I’m not sure I agree there. I go back to the OG THX speakers, when narrow verticals was a big part of the spec so most of them had a line of domes. All of them that I heard (ok, KEF and Snell) sounded off in the treble IMO.
Also, I wonder if the NFS and fixed distance measurements correctly capture vertical directivity of long tweeters. The points captured don’t vary much in distance from the speaker. I’ve always subjectively found long tweeters to fare worse in the “sit-stand test” than calculated verticals would suggest. At a 10’ listening distance the theoretical vertical listening window of a point source with 20 degree directivity is about 3.5’ if my math is right. Yet I’ve subjectively found - never measured, admittedly - treble disappears with long tweeters when you move up much less than that. But narrow horns aren’t as bad in that respect (though I’ve never used one that narrow).
With regards to the overall sound though, for me it mainly comes from the idea that I've read attenuating vertical reflections (I think the ceiling mostly, and as opposed to other wall reflections) probably has the biggest positive impact on the sound. And in my own experience, a high ceiling does wonders for acoustics. When I moved apartments, the change in sound quality from having 14 foot ceilings to 8 foot ceilings was dramatic, despite having a wider room in my new place...
Overall, like most things in audio, I've gathered both positive and negative impressions of narrow vertical directivity. Usually though, when I do see people complain about it it's about the sweet-spot (as you mentioned) rather than overall sound quality. The BMRs are still quite loved, after all.