think winter when frozen (we all know dry treatment wears off) ropes cant get through some of the belay devices
the other reason is that it works perfectly fine .... if you dont lower or rap on the munter (dont weight it continuously) it really doesnt twist the rope
but the best reason is to keep in practice because when you do need it, or the munter mule for a tie off ... you wont spend 20 min fiddling and remembering how to tie it
a few years ago we were on something like the 8th pitch up the chief ... and i hear this CLANG CLANG CLANG down the rock from my partner at the anchor above... seems like he dropped his belay device ... he couldnt remember how to tie a munter ... good thing i dont fall on easy ground ... i lent him my atc and used the munter ...
a few times every year i go out and do some "old school" climbing to keep in practice with the basic skills .... no belay devices, nuts and hexes (no cams), carabiner brakes, rope anchors, etc ... like anything else, i can read and post on the intrawebs all i want about it, but without PRACTICE, PRACTICE, PRACTICE its all for nothing
heres a good article on the munter by one of the top female american guides