With the Wyeast smack packs, it's essentially the same thing. If the packs swell, you know you have viable yeast, so the starter is just there to increase the cell count.
With White Labs vials, you really have no way to test viability other than to make a starter.
Two packs/vials is about right for an average-gravity (1.050-1.060) ale, BTW. For higher-gravity ales or any lager, you'd need more.