I personally still think series should be used for items that have an intentional, chronological relationship to each other. Like multiple volumes of a collection, monthly magazine discs etc.
Just grouping some otherwise unrelated items together because they share some common characteristics should not be a series. This is better put into a collection (which is just an arbitrary collection of things someone wants to group) or even just done by folksonomy tagging.
For example a label might put out collector picture Vinyl releases under a common theme for their artists. Those releases are marketed and designed in a similar way and probably share some common naming. This could be a series.
But if someone wants to just group all picture disc releases that exists (e.g. because they are collecting this specifically) this is not a series. There is no deeper, intentional relationship between all picture disc releases in existence. Rather this could be a collection or indicated by a folksonomy tag.