You never know if that reliable source will still be online tomorrow.
That’s true. Wrong info on the cover art should always be documented in the annotation.
Of course, we cannot strictly request that. But still, better have the full art work that not. If some label does not allow them being online … well … then their releases won’t be “high data quality” on musicbrainz.
As already mentioned elsewhere, I’m in favor of adding an additional quality level “very high”. Then “high” could be “all available data added” and “very high” should be “all we can dream of” (including full cover art).