Will Japan, if it decides to join the US in any Taiwan scenario, be a valuable asset (for the US)? I mean I looked up its naval equipment and I was not too impressed. It only has 8 full sized destroyers with 90+ VLS that carry standard missiles. The rest 28 smaller destroyers only have 32 VLS that carry the quad packed ESSM, with 50km range. These smaller ones have to get awfully close to China's mainland to have an impact.
They don't have to.
Those smaller DDGs (officially known as "general-purpose DDGs") are mainly tasked with escorting the larger surface warships (i.e. AEGIS DDGs and so-called "helicopter carriers" in the JMSDF), with primary focus in close-to-medium-range anti-air defense and/or anti-submarine warfare. Anti-ship and anti-surface strike missions are conducted by the AEGIS DDGs (i.e. Kongos, Atagos and Mayas) with their Type 12s and Tomahawks instead.
As for the older DDGs and DEs (i.e. the Asagiris and Abukumas), they will be replaced by the Mogami FFGs by the end of this decade.
Maybe its strength lies in its excellent submarine force? Or maybe its air force? I used panic at the thought of a combined US+Japan+Korea involvement in a Taiwan scenario. But when you look closer at their equipment it does not seem so scary.
The JMSDF operates plenty of pretty good if not state-of-the-art SSKs, that's a given. Though, given Japan's geography (surrounded by ocean from all sides) and the need to focus on multiple vectors against perceived threats (not just from China, but also North Korea and Russia) that can span 1000+ kilometers apart - There's only so many places that they could pool their SSKs to cover them.
The JASDF is also a considerable aerial enemy to deal with. However, many of their mainline fighters, i.e. F-15Js are getting old, overused, and increasingly in dire need of being replaced by the F-35As.
Maybe Japan's most usefulness is in its military bases, able to provide the US with a place to refuel and re-arm.
That's one way to look at it.