The only case where the less interference / additional bandwidth of 5GHz makes sense for streaming is if you have a file server on your local network that holds all your HD content and you stream from it wirelessly. Netflix streams (or any internet source) are coming to you over your internet connection, so their bandwidth is already constrained by the speed of the connection to your provider. Your wireless network (regardless of band) has at least 10x the throughput of your internet connection.
The primary real-world benefit of a simultaneous dual-band router is that you can run devices that only support 2.4GHz at the same time as devices that can use 5GHz. Older routers that supported both speeds would have to step down to only using 2.4GHz if there was only one device on the network that required it.