in japan bitcoin is already very big they use it like they use their jpy or yen, it's hard to think that it would make any difference there in japan, if it was another country where bitcoin is not very known perhaps yes, but nonetheless it's a good thing more exchange more possibility to have more people that buy bitcoin
No one in Japan is using it. It is legitimate now, but it's never been used anywhere like the same way as in the US for instance yet. Bitflyer, currently the biggest exchange in Japan, has most of its volume due to less than ten traders.
And if this does become a big deal no one will be using it still. All they'll be doing is trading it.