My guess is that the beta needs work.
The beta Rawk3 that we got to test was indeed a miracle of Wii hacking achievement - the idea of being able to stream thousands of songs from a computer via wireless seemed ludicrous until I tried it and it worked extremely well. That being said, there were a lot of bugs left to fix.
The Wii app runs great, but the SD and USB device compatibility isn't perfect yet. The computer app, while a major step up from the Rawk2 program in terms of interface and usability, is rife with little bugs.
Additionally, because of Riivolution hax and the potentially large song list the system must keep track of, "Rawk Band 2" consumes a lot of the Wii's running memory, making the crashes that plague Rock Band 2 Wii far more prevalent, and with the slightly-longer (perhaps 10-30 seconds) time from powering on the Wii to playing songs this means a lot of downtime. Playing songs via Rawk3 was pretty awesome, but random crashes and (depending on storage format) occasional gameplay judders made party play - both local and online - somewhat tiresome.
I'm not a dev so I can't speak for how many of these issues are fixable or planning to be fixed. I just bring it up because the amount of work justifies the time it's taking.
Rock Band 3 should tide you kids over for a bit anyway.