Most of the bikes you see on here with only a front brake are fixed gear (no coasting) so in effect, the pedals double as a rear brake. Running a rear only on a SS would be roughly equivalent to riding fixed with no brakes or a SS with a coaster brake only. Certainly, lots of folks do both of those things.

The argument for having both is similar to the argument for running a front brake with a fixed gear. There is safety in redundancy. Should something happen to your ability to brake on the rear wheel, the front is still an option. If you're comfortable putting all your faith in a single brake then that's fine. Lots of folks get by with a single method of braking everyday.

That said, I'm putting 2 brakes on my SS build because I don't trust them to be 100% reliable (and because my frame came with 2 brakes and some cool holders for the rear cable welded to the top tube that I like the look of with cable in them). My decision is based 80% on function, 5% convenience, and 15% looks.

Your decision is totally up to you.