If my math is correct, max average bitrates of a single BD-50 for each movie, including BOTH audio and video would be the following:
Fellowship of the Ring - 33 Mbps
The Two Towers - 29 Mbps
Return of the King - 26 Mbps
While bitrate is not the be all end all, I think Two Towers and Return of the King are pushing it for something that has no reason not to have reference audio and video. I'd rather seen an average bitrate at at least 32 - 35 Mbps, but if the reviews are positive and they fix the DNR issues of the first release, it won't be a deal breaker.
Assuming the extras are straight SD ports of the previous extras, I'd rather just see a 6-disc set. Give the movies additional room to breathe on 2 discs per film, still have room for all of the extras in the remaining space.
I guess they put the extras on DVDs for those who have not upgraded to blu-ray. Wait...what?! 