Adding in my support for this. My home has multiple Access points running at 80Mhz channel bandwidth. Because I have more than two access points, I need to run additional APs on DFS channels to avoid interference between them. Although I know the Roku does not support 80Mhz wide channels, it is beneficial for other devices since they can get off the air sooner, giving breathing room for the Roku to enjoy. I do have a Roku Streaming Stick +.
I also administer a number of apartment building networks, where the building construction and tenant usage patterns (gamers, students, streamers, etc) require a high number of access points, and I run the APs at 40Mhz. Because other apartment complexes are nearby and sometimes tenants bring their own network, I also utilize DFS channels there. The tenants utilize a lot of Roku devices, including ones embedded into TVs, and they are unable to enjoy the performance benefits of having 2/3rds of the 5Ghz spectrum wide open for use. I'm sure many of the existing products can support 5Ghz DFS with a software update, but they may require recertification with the FCC to complete. Either way, one competing device, Apple TV in particular, supports DFS. Most other streaming products like Fire TV, NVIDIA Shield, and Google Chromecast, don't support DFS. This would be one of many ways to differentiate Roku from these other products.
It's worth pointing out that many consumers also use ISP supplied routers and Mesh systems, and many are starting to utilize 5Ghz DFS as the other 5Ghz channels start to get too congested to handle all of the devices, especially in densely populated urban environments. Verizon FiOS is one such ISP whose routers will select DFS channels on a regular basis. Business users with higher end wireless systems will also utilize the DFS channels for performance.