Joint stereo limitations:
Mp3 can not switch joint stereo mode for specifics scalefactor bands. If joint stereo is used, it has to be used for all the bands. This is rather inoptimal, and is limiting the use of joint stereo. As an example, imagine the following situation:
The lower frequencies are featuring an instrument playing on the far left, and frequencies around 1500Hz are featuring a singer in the middle of the stage.
In such a situation, it is not possible to use joint stereo with Mp3 because of the lower frequencies part which is too different between both channels. A further bitrate reduction could have been achieved if it was possible to toggle joint stereo mode on a scalefactor band basis. (in this case regular stereo would have been used for the lower frequencies, and Middle/Side stereo for the remaining part of the frequency spectrum)