Run-ups are put in races as an added challege. The hill is not steep enough that you couldn't ride up it. It's just faster to run up it.
That said, I usually choose the gear that I can ride up the steepest hill, but not spin out so fast on the flats. If it's flat, I'll ride around 2.1:1 (55"). If it's steep, I'll ride 1.6:1 (41"). I ride with a cassette hub and a bunch of cogs.
One of the reasons I like riding singlespeed is that it forces you to find the flow of the trail--that point where you're maintaining or gaining speed--without pedalling. With proper technique, you can accelerate out of corners without ever pedalling.