In three ways:
First, if we assume that japan remains an ally of the US, and historically this is a valid assumption, then a strong ally is good for the whole alliance.
Second, if we assume that Japan and China are not allys, and historically this is a valid assumption, then is a strong Japan good for the US? Yes, if we believe that the US and China are economic rivals. The rivalry is debateable but common theory would suggest yes in areas such as technology, labour/manufacuring and access to finite resources. So a strong Japan could signal a weaker China.
Third, and lastly, for years Japan and the US were friendly politicaly but fierce economic competitors. This dichotomy was noble on the part of the US who could have shut down Japan militarily or through massive trade sancions and other alliances globally. To add historical persoective, this serves to the US credit that they assisted in the re-build of post-war Japan, honorably engaged with a trade rival and ideally, should Japan be an enduring economic power, the US will have achieved the ratest of modern feats- delivered a long-standing, freely ruled, safely governed, democratic ally after having dropped not one but two atomic bombs on them. It would be a remarkable achievement and emblematic of the enduring power of economic alliances with the US.
The political currency and economic goodwill would be immeasurable.
