User blog comment:Hayashi H/Advanced Equipment Guide/@comment-85.76.165.235-20151228200037/@comment-26171095-20151229070835

It's because you need the 10cm + AAFD for your DDs actually participating in the combat. The support shelling equipment is basically all your leftovers. If you have excess 10cm + AAFD, then sure, use it. If not, then 120mm are the best thing to slap on because they can't be upgraded, so they're only really good for support expeditions anyway.