Well, if you can't afford not to, you can't afford not to. Period. The author is simply stating that of all of the variables you control in choosing clientele,

Well, if you can't afford not to, you can't afford not to. Period. The author is simply stating that of all of the variables you control in choosing clientele, making sure they aren't assholes should be near the top of the list. It's a better choice to take a lower-paying gig with a reasonable person than twice your rate with an impossible client.