If i wanna think of worth doing as meaning getting a return on investment, i have the right. However, my thrust here is that getting started on the asteroid belt is an exercise in doing something practical in space, which yields something more than bragging rights or rather passive micro-gravity studies. The asteroid belt may well be, probably will be, an important resource in the future, and it can also benefit space exploration because if handled well, it can atrract corporate sponsorship. Once that happens, nobody any longer cares if its "sexy" to the tax payer.
Mercury, Gemini and Apollo were great show pieces and inspiration--they were also a wonderful way for us to catch up to the evil Russkies in the development of rocket payload boosters. Not only that, by making it about publicly broadcast manned missions, the program perforce produced a high degree of accuracy in control of the rockets, as well as developing sophisticated telemetry--all with a military end in mind. Because Kennedy dubbed it the new frontier, you could get the tax payer behind a multi-billion dollar program which would have had far less appeal than telling the taxpayer "Sputnik scared the **** out of us, and we desparately need to catch up to the vile Commies in building reliable ICBMs."
These days, you're going to need corporate participation to keep zipping around in the great vacuum, and for that, there needs to be a pay-off.