The point is that the first 150 or so miles of space travel -- braving the gravitational well of Earth and shooting through the atmosphere -- is the most difficult and dangerous; the next million miles are comparatively easy. Yet going up and down that first 150 miles is the least glorious, least inspiring of all space adventures; it is the stuff beyond low-Earth orbit that speaks to our yearning as a restless, seeking species.
Of course, the long term solution to getting up the first 150 miles, or actually the first 25000 miles, is to build a space elevator. If such a thing can be built, then this will solve the two problems inherent in using chemical rockets to take people to and from orbit: specifically that this is extremely expensive, and that it is dangerous.
For now, we need to keep the shuttle going because we have no other way to get into space. And we'll need to support the space station for a few years, because we have no other program in place.
But that is not our destiny, nor our purpose. If we're going to risk that first 150 miles of terrible stress on body and machine to get into space, then let's do it to get to the next million miles -- to cruise the beauty and vacuum of interplanetary space to new worlds. Back to the moon. Establish a lunar base. And then on to Mars.
The Columbia tragedy will give voice to the troglodytes who want to give up manned space travel altogether. But the problem is not manned flight. The problem is this kind of manned flight, shuttling up and down at great risk and to little end.
However misguided the shuttle and ISS are, shutting these projects down now would seem to be giving in to the troglodytes: it even if this was not the intention of shutting them down, it could end up being the result. So keep them for now, but only for now.