It isn't actually intended to be used to replace the core ships (and it really isn't--the only part of it that lets you do that is defining a new prefix). The purpose is for assigning templates to the various types of ships the AIs design, so they use them appropriately. That is why it is only available when customizing an opponent.
Yes I understand that, and I'm not asking for a tool that lets you replace core designs. I'm simply saying that, as it were, anyone who wants to customize the races' will just have to do it with more needlessly clicks.
I consider a bug the fact that a style can work for an AI but not for a player. But the fact that the button is disabled is more like a design quirk that could be improved.