Used to figure you could just swap one paste for the other and call it done, but this exact kind of thing changed my mind on that real quick. The mix is the way to go, something like 3 parts PVA to 1 part wheat paste keeps a little give in the spine so it still opens without flopping around. Straight paste has its place on paper hinges and corners, just not on a whole spine you actually need to move.

This is the same thing as buying those $5 sunglasses at the beach that snap in half the second time you wear them, or those gas station phone cases that crack if you look at them wrong. You know it's junk when you buy it, but you're in a pinch so your brain goes "how bad could it be." Then it's bad. Same deal with cheap extension cords that get warm, knockoff power bricks for laptops, those dollar store charging cables that stop working after a week. The savings never hold up because the cheap version just breaks your more expensive stuff.