Many horses at my barn benefit from them!
I think they're very useful for horses that need them!
When I use ear plugs my horse is completely different! I use them every time I ride!
They're great! Muffling the sounds of big crowds reduces stress and distractions. They just cannot be left in for to long (ie several days) to avoid infection. I ride in a musical ride, preforming at agricultural fairs and downtown in cities and they make a world of difference.