Should I buy a JRP? I need advice.

Ok I was getting ready to purchase my JRP and my sister who I am going to visit in Japan told me not to. I don't think she's familiar with the JRP because she doesn't qualify for it. But for me it sounds like a great deal. Unfortunately I won't be doing much traveling when I'm there because my sister is pregnant and I will be helping her with the baby. But I told her within the two weeks that I will be there I will definitely take a day or two and explore Tokyo. She told me that the JRP is for people who will be using the train a lot. Let me break down where I will be using the trains as best as I can...

From Narita airport I will have to take a train to the Tokyo station, where I will then board the Shinkansen towards Aomori, but get off at Hachinohe where my sister will pick me up. She is stationed at Misawa AFB. The round trip cost for just getting to and from Hachinohe is already about $350 I believe. And if I plan to go and visit Tokyo I would have to make that trip again for another $350 correct? So that is a total of $700 where if I buy a 14 day JRP is would only be around $580. Plus it leaves room for if I want to travel anywhere else.

I hope this makes sense. I think I should still buy a JRP but I need an opinion from someone who knows more than I do.

Thanks!