There is no doubt that they are pure enough. Working out of a kitchen lab, it is unlikely you or I would ever be able to replicate the packaging sanitation systems that Wyeast and White Labs have.
That said, whether to culture straight from the vial/smackpack or to build a starter first represents a bit of a tradeoff. Culturing straight from the vial wins hands down for culture purity; building a starter first gives the yeast a chance to build up glycogen and trehalose reserves that may or may not have been depleted since it was packaged. If the yeast package is old, or if it has been mistreated at all, I would definitely build a starter. Otherwise, it's a judgement call...I tend to go straight from the package if I trust the place I bought it from.