The presence of a Japanese military would be significant in stabilizing the region. While there is not a great lot of trouble in East Asia, the region is not without problems. Piracy, kidnapping, religious violence, organized crime, human trafficking, and more are among the problems in the region. If Japan increased military presence in East Asia, the region would be more stable.
The possibility of increased tension with the middle east means the U.S might have to redeploy it's navy in the Med & Persian Gulf regions, however we can not risk weakening the pacific area either so we should build up our allies like Australia, New Zealand, Taiwan, S. Korea, Indonesia and Japan so they won't be so vulnerable if there were sings of a pending conflict with either PRC or DPRK while there is a major one going on in the middle east at the same time.