It seems to me that the continued existence of NATO is largely a reflection of the incomplete consolidation of the E.U. Since "Europe" is still only a loose collection of a couple of dozen countries, each with an independent foreign policy and defense policy, it's wide open for NATO to provide a unifying structure--one that happens to be mostly controlled by the U.S.

The weird thing is that the U.S. has more economic ties to the Pacific region now than it does Europe. http://www.census.gov/foreign-trade/statistics/highlights/top/top0812yr.html
So there is probably not a really strong underlying reason for the U.S. to keep NATO going. We would be better off spending our energy working with China and Japan to get our relationships in that direction under control, and let Europe worry about Europe. But since there's no big push from Europe to develop an alternative, NATO continues...

A quiet and passive thrall is the best type to have.