Well, I cannot speak for others, but here is the reasoning behind my conversion:

. My Motobecane Mirage mixte was not a high-end or rare or historically significant bike, but a very common, mid-range mass-produced model
. Its geometry and handling made it appropriate for fixed-gear conversion
. It actually feels much better to ride as a fixed gear than it ever did as a geared bike

Motobecane Mirage mixtes were not very comfortable bikes. The geometry is way too aggressive for the women they were typically sold to in the late 70s-early 80s, and many of these bikes have basically spent the past 30 years in basements and garages entirely unridden, before they were sold off on C-List during the recent mixte craze. Now young women are buying them, putting upright bars on them in attempts to make them into cute city bikes... but they still aren't comfortable due to their twitchy handling and steep seat-tube angles. If anything, turning these into fixed gears allows the owner to get the most out of the way they were designed. In my opinion of course.