I guess it depends on how you get enjoyment out of the game.
A lot of people (like yourself I'm assuming) love RB because of the overall immersion-aspect that comes from playing it.
Myself on the other hand, see RB as a way of better enjoying the music I like to listen to. Essentially, it's interactive music listening. Thus, a very awesome outro, such as in PPLL, being covered up and having me play at-random, would be pretty disappointing.
As it stands now, BRE's are to increase score (which I don't care about) and allow free-style endings.
I play drums. I would agree with you that BRE's would be pretty cool...
the sounds associated to each pad changed according to each song (as they now do in GH games). As it stands now, you have the same 4 sounds (w/ standard sets) in every single fill and BRE, and most of them time they don't even fit the song your playing.
ANYWAYS; this is completely off-topic, lol.