Ultimately it's a matter of personal preference as to which material to use. If I was storing my boxes in an attic or cellar, or still living in Florida, I would probably want to use plastic, but since they're in closets that are already well-regulated heat/humidity-wise, cardboard's more than suitable for me.
This guide isn't meant to be only for those who prefer cardboard over plastic; cardboard boxes are used in the examples mainly because that's just what I use.
(Also, I've been collecting in general for about 25 years. Pretty much started with comics, many of which I still have, stored in plastic bags and cardboard short boxes ^_^)