well, depending on the dates that you plan on riding the shinkansen, it might be better to get a JR pass for a shorter duration of time. If, for instance, you'll only be in Hiroshima for a week, wouldn't it be a better value for your money to get a week or two week pass, just to cover the period of time between your rides on the more expensive train?
Also, bear in mind that the JR pass only covers JR transportation; I don't believe that this covers the subway or other company's trains/busses/etc. If you don't plan on doing a whole bunch of travel other than your rides on the shinkansen, I'd say plan your purchase accordingly. But yes, a JR pass would definitely be a better value than a round-trip ticket.