The current role for NATO for the US is purely geopolitical. Militarily Nato outside of the UK and France will never be able to supply much outside of soldiers on the ground. Germany could do more but is limited to a defensive role. Canada of course has too small a military to make much of an impact.
As for the future of Nato, I think it will be rather limited. France and the UK will be making cuts to their military, (especially the UK) that will impact their ability to operate large distances from their boarders
Overall without a real common threat and no a rag tag bunch of terrorists do not make for a significant common threat requiring hundreds of billions of dollars in military spending, the goals of the US and European nato members will diverge. Leaving the generally by itself to counter China (yes China) which is the main issue for the US military in the future.