we must keep a presence in the faces of our former enemies. the best reason i can think of is there must be a realization to them that we are not there to take over, but to keep the peace.
a benefit for the American people? you're damn straight! this presence also gives our future enemies a place for them to concentrate their efforts on someone else's turf. if we abandon all the people we have liberated in the past, the people we kicked out will just come right back and take over. Taliban, etc...

if we leave anywhere and try to lock down our borders it will be nothing more than an invitation for terrorists to come here and try some bullshit. locked down or not, there are many holes to fill before we could even think about this course of action. with all the dumbass pacifists around it will be the beginning of our destruction. we are already on a spiral out of control towards implosion, we don't need a kick in the ass to expedite our destruction, our own citizens are trying to do it for them.

i blame the fall of the roman empire on the roman senate. the people don't know shit! they never have and they never will.