I could not agree more, which is why when a player selects one of these classes they have the advantage of a ready made reason to be an adventurer and risk their lives to do heroic tasks no ordinary mortal would dream of doing. The other classes would have to figure out what would motivate them leading to conversations with the GM that might take a lot more time. The Paladin, Cleric, and Warlock come with ready made reasons which the players can contour to meet their own goals.

A GM that wants to build a story should use these ties to enrich the narrative and add complexity and depth to the groups dynamic and arc. In a story, the player and GM can use these fluff ideas to build on, or they could both ignore the fluffy bits and just focus on the mechanical advantages/disadvantages. The 'SHOULDs" are really complaints about playing at a table where the group might be ignoring or sidelining some juicy story fuel which can be very frustrating.