Paul Spudis says that propellant depots are a necessary but not sufficient condition for opening up the solar system.

Well, in the long run, sure. But as Clark Lindsey notes, in the short term, I think that a dollar spent on reducing launch costs will have a lot higher ROI than a dollar spent on getting propellant from the moon. That’s just the harsh economic reality, largely because reducing launch costs is a very low-hanging fruit, given how ridiculously and unnecessarily high they currently are. Elon has already started to show the way, and fully reusable space transports that develop out of the suborbital and other markets will accelerate the process. Once we solve that problem (and it won’t take that long, once we get serious about it, which will start when the markets flower), then ISRU will start to look a lot more attractive, because doing it will be a lot cheaper as well.