Xel smacks him on the top of the head again, harder. "That's a last resort!" he scolds. "You won't have their help later, and sacrificing an ally at the first sign of danger creates a reputation that gets around! Also enemies, if they have more survival techniques than you do!"
He'd use considerably stronger words if Rincewind were really the wolf-pup being readied for patrol work that Xel is currently sort of treating him like, but Rincewind isn't mazoku, so enlightened self-interest will have to do.
"You've been running for what, three minutes?" Xel scoffs. "I thought a flee-er like you would have more endurance. And it's completely fair: you accepted a teacher and are defying his instructions." He pats his wrist, though, without letting go. "If you have an objection to a task, ok-desu, you don't understand it. So ask what it's for, in that case, unless you were told to do it quickly--but ask, don't whine."