Yes, ion engines might be possible, but the question then becomes keeping those maintained, and that is expensive. Nothing in space holds up for extreme amounts of time, and whats the point of keeping an orbital junk pile? People keep saying historians of the future, but think about the chance for space trash. If we just leave that thing up there, the chance for a collision is greater than zero, so we are risking another source for the ever expanding amount of space trash. (http://en.wikipedia.org/wiki/2009_satellite_collision for an example). The more space trash, the riskier space flight becomes, the harder it is to keep up the budget.
I'm all for keeping the ISS up and running for as long as its useful, but the idea of a flying monument to ourselves is arrogant and wasteful. If it's not generating science, then its sucking money from projects that could be, and that's NASA's goal, to generate science. An ion engine would be expensive to develop (really, there are only prototypes), expensive to transport to the station, expensive to attach to the station, and nearly impossible to maintain without someone on the station, which would then mean that it might as well be manned fully to complete science. If its manned fully, then there will be Progress launches to keep them supplied, and with a Progress up there, you've lost you're reason for an ion engine. That didn't need any long division, now did it?