Horn based loudspeakers why the controversy?

Indeed. Zu are awesome speakers. 

If you have an obsession with certain kinds of amps, SETs and laterals for instance, you really need horns to get the best of those. In the rest of the world of solid state gain is very cheap and you can accomplish dynamics with raw brute force easily and cheaply. Efficiency just isn't as important as it used to be. 

Horns sound more like real musicians than conventional speakers, to me at least.

 You cannot get the same dynamic range with inefficient speakers. You can apply as much power as you want but you will not get as great a range between the quiet and the loud as you will with a more efficient speaker. 
That is why typically efficient speakers will sound more realistic at lower volumes.

I'm a fan of horns, though I currently don't run a pair.

I have some close friends who REALLY dislike horns, and instead of trying to convince or talk over them, I prefer to listen and understand their complaints.  Beyond the typical complaints about shout, some centers on the integration or lack thereof of such disparate drivers.  Even as a fan, I acknowledge and understand, though I've listened to more than a few "regular" designs that do as badly or worse.

One more thing...horns come in both front-loaded and back-loaded configurations.  Almost always in these discussions, we mean the former when we use the terms generically, but some excellent back-loaded horn products have come down the pike

 Everybody has horror stories about hearing horn's. I remember the first time I went to addition a pair of model nineteens. The owner had them hooked up to a solid-state Mac and proceeded to turn them up so they were playing at about 120 DB steady. He looked at me with a big smile  and said see how clear they are? He didn't even have them set up properly, had  them facing one another across the room. I couldn't get out of there fast enough. Thankfully down the line I had some reasonable experiences  and grew to appreciate horns greatly. 

 Regarding Altec A7's I had a pair for a while and after quite a few mods they can sound quite good.