It certainly isn’t useless, because it forces your opponent to consider what could happen when you use them, which leads him to leave his carriers behind, which during sea battles, are awesome for soaking up free hits. Its also “piling on” since japan doesn’t usually need the kami to defend the homeland (it can do it fine the conventional sea/land way up until the climatic battle) but it still gets it.
I said next to useless, in a huge naval engagement sure you get 3 extra hits before combat but that isnt as game changing as the dice are in these situations.
For defence well the allied player just sends air + transports and it can non combat the fleet later, and if you put a big enough fleet there then its just a big naval battle with 3 hits before combat ( if your lucky )
It can be usefull if you have a sub in SZ6 as then you can kami the destroyers and then suddenly the transport isnt escored anymore and cannot unload ( but no combat so no retreat either )