I'm a realist. Our power only exists if we can use it, and we can only use it if we have reliable staging areas. For instance, Rammstein AFB in Germany saved tens of thousands of soldiers' lives, as the primary channel for CasEvac from the Middle East. Without Rammstein, we lose every war we fight in SW Asia or Africa. That means we have no power over SW Asia or Africa. That means we have no bearing whatsoever on what happens there -- and call me crazy, but I kind of like having clout.

Take Japan. Are we OBLIGATED to defend them? Actually yes, by the terms of their surrender and numerous other treaties, we are obligated, but pretend we're not. What those bases give us, is the real right to exercise power in the Pacific theater. Nobody goes to war in Asia while we own the Pacific unless [i]we[/i] say so. So what has that gotten us? $300 HDTVs, for one thing, and reliable shipping to the world's biggest trade hub in Singapore. 

TLDR -- US bases overseas aren't a cost, they're an export. They're a large component in a global trading economy, which enables basically the entire economy to function.