I use them with shedding horses with thicker coats, or with caked on mud, but otherwise I don't really use them. The rubber curries are more effective at massaging the muscles and bringing out the natural oils that make your horse's coat shiny.
I rarely ever use them. I believe that few horses, like them, you can scratch their skin without knowing and they break the hair. If you groom your horse properly daily, a rubber curry will do just fine even with caked mud😊
I use metal currys whenever a horse is shedding. There isn't really an issue with using them year round as long as you don't press too hard while they have short coats