It seems like a good idea, extending the auxillary lane several hundred feet past the exit so as to smooth out weaving, why is it only used predominantly in Minnesota, and in what cases would it not be optimal?
We talked about this over in the Washington thread a few weeks ago: https://goo.gl/3zfeiX
It's also very common in Washington. The idea is to prevent traffic from flooding the non-exit-only lane. Because exit-only lanes have no merge area before the exit, it's up to the driver to be in the correct lane prior to the exit. Some drivers wait until the last second. Others just avoid the exit-only lane the moment they enter the freeway -- that last one is the real issue. If the lane merged left, drivers wouldn't feel completely obliged to be in the non-ending lane miles in advance. As a result, you get more capacity out of what would otherwise be a poorly utilised lane.
MnDOT layouts also call such a lane an "escape lane". In particular at cloverleafs, it gives those entering from a loop ramp a few more hundred feed to speed up and merge.
I found the same approach used extensively along I-27 in Lubbock, Texas, when I visited in 2004.