Bah, you don't need foot retaining garbage! I happily run trials style pedals on my SS MTB for maximum traction, foot or shin!

I run clips on my fixed roadie and track but ran beartraps on those too for some time. Some people are afraid that given the inability to freewheel on a FG, you're pretty screwed should your foot fall off the pedal (the cranks get ahead of you or you have a hiccup in your pedaling that causes your foot to slip off). They may be right, but I never really had a problem.

In short, ride what you like. If you find yourself thinking you'd really like a more positive response from the pedal or you'd really like to be able to pull as well as push or spinning a high cadence becomes a hairy scene, well that's the time to look at foot retaining systems.