Under the current git version of xfce4-display-settings, if the user wants to automatically enable display profiles when attaching a new display, they must first/also enable "Configure new displays when connected". I want to have my display profile automatically enabled when I connect my external display but I don't want to use, see or have to close the displays window that appears every time I attach my external display.
Understandable. In short that means patching in profile support into the daemon (which currently doesn't check for profiles, but simply tries some heuristic of mirroring or "remembering" previous settings). For now, only the minimal dialog "knows" about the profiles and can enable them.
This has been fixed as part of 4.14.