Citing a Der Spiegel interview with Condoleezza Rice on the first Bush administration's insistence that a reunified Germany remain a full NATO member, Greg Scoblete makes a good point about the enduring rationale of the alliance:
The logic at the time of the fall of the Soviet Union, in both Washington and Europe -- including, significantly, in Paris -- was that a continued American presence in Europe was essential to guarantee a stable transition. Back then, remember, reuniting the two Germanies was an anxiety-causing proposition, let alone the idea of integrating all of the former Warsaw Pact countries into what was not yet even the European Union, but the European Communities (until 1993). So the mature institutional structure of NATO and the American commitment it represented was a welcome insurance policy, and one whose dividends paid off in the Balkans and Kosovo.
Of course, as the Soviet threat and the instability of the immediate post-Soviet European environment recedes further into the historical past, it's easier to question whether NATO remains relevant, as Stephen Walt does here, or even effective, as David Ucko does here.