It defaulted to "off" because it's an entirely new toolbar that by default is not visible unless something (other than items in the compatibility shim) has been put there by an extension.
This is not true. It is not visible, full stop. Attempts to have the addon bar appear automatically if something is placed on it (or the status bar), either by a new extension or manually by the user, have essentially failed. Go ahead, try it, the bar won't appear on its own even if you drag a button to it yourself. A small subset of (restartless) extensions that place items on the bar will cause it to appear (or used to, I haven't tested recently) but in most situations it won't. This remains the case as of b12 and I don't see anything coming to change it. So 90% of users will never see it, not because they don't want it, but because they don't know it exists.