VBR have been the red-headed step-child of MP3 since the beginning. I have no idea why, maybe it was difficult to write good encoders and decoders for it for back in the day. The few times I used them, I found trivial space savings compared to a CBR and to be honest I thought I could hear a difference (maybe its not humanly possible if the implementation of the encoder/decoder is good but what if the implementation is sub-par?). Storage and bandwidth are so cheap, why bother today?