To test how well Roku can handle high bitrates, I created a test 720p video with the MainConcept H.264 encoder using full Level 3.1 High Profile at a bitrate of 17,000 Kbps (near the Level 3.1 High profile maximum of 17,500 Kbps), and my Roku XDs played it without apparent difficulty.
Why then does Roku recommend limiting bitrate to 12,000 Kbps? My guess™ is that playback problem have occurred at higher bitrates, but that such problems may actually be caused by slow USB flash drives, not higher bitrate per se. The USB flash drive I used for testing (Patriot Supersonic Xpress) is exceptionally fast with read speed (as measured by H2testw) of 28,000 Kbps, so it wasn't a limiting factor.
My conclusion is that you should use a fast USB flash drive with read speed higher than 17,500 Kbps.