If you're riding a lot of hills, two brakes are good for a variety of reasons. The obvious one is that a certain amount of redundancy is nice when the alternative is careening out of control down a mountain.
But moreover, having two brakes means you can alternate them on a long descent and not overheat your front rim. It also means that you can distribute the work between both hands/arms, which is nice on long rides. I've done up to a hilly 400k with only a front brake, and my left arm was MUCH happier when I put a rear brake on as well after that.
Simply controlling your speed down a long descent isn't a scenario of absolute maximum stopping power (which is the usual argument for a front brake over no brakes at all on a fixie); you can scrub of some speed using either brake. But there are good reasons for having both if you ride in hills.