Finally. Roku announces on their blog (http://www.tomsguide.com/us/roku-smartphone-pc-screen-mirroring,news-19674.html) that the Roku 3 will soon be able to mirror just like Chromecast and Airplay. The software updates are rolling out but may take a few weeks to get to everyone (I cant wait).
Roku also provided step by step instructions to make sure your hardware can handle the new mirroring feature here: http://support.roku.com/entries/56266670-How-do-I-enable-screen-mirroring-on-my-Roku-player-.
Let me know how it works out for you…