Don't agree with shrimpx: even with good maintenance, things fail & it's useful/safe to have a back-up. Chance may be low but consequence is high on these failures so extra back-up is worthwhile.

However, 2 reasons for me:

1. long / steep hills: having to ride 1km at 180rpm (as I had the other day) is not an option. At that cadence I just couldn't brake with my legs smoothly enough.

2. The law. UK law (which is similar pretty much anywhere) says that a bike on the road should have 2 indepedant means of braking: a fixed cog is good for one, brake is the second.

In fact, I've 2 brakes on my as I fitted a flip-flop & I prefer the balanced look it gives: sometimes the rear is useful even with the fixed for braking down those hills.