Addons should only be needed if the functions aren't added by the devs, proper inclusion from the devs of a simple clean UI and tons of options to edit it, clean dps meter/parsing, different options for controls.(cast aoe abilities on button release/on press, option to heal/buff/damage targets by hovering over the party/raidframe/unit with option to choose default target if not hovering over someone) )
downside of allowing anyone being able to create addons is alot of stuff going too far e.g: addon telling you what monster to burst first in mechanic where one needs to be killed first and without the addon the visual indicator would require someone to call it in voice/text. If too much manual calculations/timings are automated it makes the dungeons/ raids REQUIRE the addons and makes the encounters easier than intended.
I think allowing custom UI addons should be fine though aslong as it shows same elements that are visible in default UI layout but anything more than UI tuning should be avoided unless added by the developers themselves.