We've tried the 'get something great as a result of spin-offs' route before. It disappoints every time. Why are people advocating it again with a human 'flags and footprints' Mars mission? Why don't we instead direct resources towards an inspiring goal which delivers something concrete?
Given the choice, I would much rather today have a permanent LEO infrastructure of a dozen privately funded Skylabs (from those same tried-and-true Saturn V rocket parts) and cis-lunar tugs rather than the 'spin off' soon-to-be-retired Goldberg contraption of Shuttle.