It's nice to have that as an option. Not everyone will enjoy, or want to deal with, older manual focus glass. I found that much of the older glass actually increased in price as people began adapting it to their MILCs, some older glass is reaching the point of being unobtanium.
Used Ai-S and non-Ai on my Nikons before departing for Pentax. The Df was almost the perfect platform, especially considering the ability to use non-Ai with no mods. Preferred their rendering to many of the newer lenses - to me, much of Nikon's newer glass is just too perfect, almost sterile.
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Currently have one "vintage" lens - SMC Pentax-A 50/1.4, which works perfectly on my K-1. But I'm not a big fan of the 50mm FOV, and I prefer the FA Limiteds.
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I've also used Voigtlander screw-mount and M-mount glass on Fuji MILCs, and enjoyed the experience. Edit - the Voigtlander lenses were not all vintage, the M-mount were new lenses.