They are not a myth. They are quite real. So are the related and similar deciduous azaleas. There are certainly not as popular or as diverse as the familiar evergreen cultivars, but they have their place.
Like the popular evergreen cultivars, deciduous rhododendrons and azaleas have been extensively bred so that they are more spectacular than their ancestors in the wild. They lack the purples and blues of the evergreens, but they compensate with bright oranges and yellows that are uncommon among the evergreen cultivars, as well as reds, pinks and whites. Some of their colors are bright and fiery, while some are more relaxed. Compared to evergreen rhododendrons, the flowers of the deciduous rhododendrons are smaller, thinner, and suspended in smaller trusses. However, these smaller trusses are often significantly more abundant, and are flashier as they bloom before new foliage develops in spring.
Deciduous rhododendrons and azaleas are not grown for the foliar color in autumn, but where autumn weather is cold enough, their foliage can color somewhat well. It is mostly clear yellow, but can sometimes turn orange before falling away cleanly, leaving neatly bare stems through winter.
What deciduous rhododendrons and azaleas excel at that their evergreen counterparts can not compete with is their fragrance. Many are moderately fragrant. Some are VERY fragrant. It is rare that such showy flowers are also so fragrant. (Most flowers rely on one technique or another to attract pollinators.)
Now that the evergreen rhododendrons are blooming, our few deciduous rhododendrons are just about finishing. Visitors often ask about the fragrance, since they do not expect fragrance from rhododendron or azalea flowers, even though they can see little else blooming in same area. These flowers are so popular that we would like to plant more near windows of meeting rooms.(The article from my weekly gardening column that is typically posted on Thursday was posted yesterday, which is why this article, which is more appropriate for Wednesday is posted today.)