"What evidence do you have for that?"
Virtually every pre-package surround sound system made. By your own admission the BR5 AV uses BR-FX in the rear. The BR-FX are not as good as BR5's, or at least not as big.
Next, the other only option in BR, is the BR1 instead of the BR2. The BR1 is very a very close equivalent to the Diamond 10.1. The Diamond 10.1 have good bass response and is perfectly capable of standing on its own as a stereo speaker.
Drawing a parallel, all indications are that the BR1 is also capable of standing on its own as a good stereo speakers.
Also note this quote of myself -
"...get the best speaker you can, but at the same time, the BR1 is a pretty good speaker too."
"...it really gets down to budget, and the BR's are at very favorable prices right now, so if the BR2 fit your budget then go in that direction, but don't be worried about the BR1,... "
It sounds to me like I recommended the BR2.
And the BR/BX-FX uses the 5" bass driver making it much closer to the BR/BX1 than the BR/BX2.
The BR1 specs -
Frequency Response +/- 3dB: 55 Hz - 30 KHz
Likely the -6db frequency response is 50hz or probably a little less. That is about as good a bookshelf speakers get.
Drive Unit Complement:
* 1 x 5.5" MMP® II Bass mid-range
* 1 x 1" (25 mm) gold dome C-CAM® tweeter
The BR2 is better than the BR1, but my point was, that the BR1 is no slouch of a speaker. It is a perfectly capable speaker as a stand alone, and as such, would make a very good surround speaker.
But again, if you can afford it, and if you have the room, then the BR2 is more versatile, and will likely have a longer life. For example, if you upgrade the system in the future, the BR2 could be moved off onto a computer or a stereo in another room and would do a very fine job of it.
I didn't say don't buy the BR2, I simply said the BR1 is a pretty good speaker, and that you shouldn't worry about it not doing the job in the side/rear. It is more than capable.
But, by the same token, if you can afford the BR2, it is certainly going to add to the experience. But then if you put BR5's in the side/rear, they are going to do a fantastic job ... if you can handle the size and the cost.
Just a few thoughts.
Steve/bluewizard