The problem with SPDs is that a mere twist of your ankle, even the slightest, can unclipped your feet off the pedals, fine for roadie, but awful for your typical fixie skidder (especially brakeless), the only advantage of SPDs is that they're cheap as chips.
There are other clipless pedals that doesn't unclips that easily, like the Time ATAC, no point in having a 'fixed specific model' when there are other decent alternative.
The problem with SPDs is that a mere twist of your ankle, even the slightest, can unclipped your feet off the pedals, fine for roadie, but awful for your typical fixie skidder (especially brakeless), the only advantage of SPDs is that they're cheap as chips.
There are other clipless pedals that doesn't unclips that easily, like the Time ATAC, no point in having a 'fixed specific model' when there are other decent alternative.
The list are here;
http://www.lfgss.com/thread9986.html