In one of my projects, I have a todo item to convert a homegrown {type:func} dictionary based dispatch to singledispatch.  It has been open for 5 months already.  I wouldn't be surprised if others had something similar: a good-enough (and probably buggy) solution that is not bad enough to justify adding extra dependency in 2.x or replacing with a 3.x only solution.

Multiple dispatch is much harder to get right or even "good enough," so I don't think singledispatch popularity or lack thereof is a good predictor for multipledispatch.