Expertise and passion
Should the person who runs the customer service operations at a ski school also be required to love skiing?
Can the CFO of a large church be an atheist?
Does the head of marketing at Kodak have to have a passion for chemicals?
It's true, "write what you know, write what you love." The commitment comes through. But does that mean that boring products shouldn't be marketed? Does it mean that the community theater must limit the list it considers for any job only to people who are 'in' the theater, who have paid their dues?
How many worthy causes have lousy operations teams? How many hobbies and sports are staffed by fans, not professionals?
I think if the work is important, it should be done with passion and skill and flair. But the work of balancing the books, or running Google adwords or making sure that customers are treated well at the ski school often has nothing to do with the product or service itself.
It's more important that you be passionate about what you do all day than it is to be passionate about the product that is being sold.
Give me someone with domain expertise and the passion to do great work any time. Belief in the mission matters (a lot!), but it doesn't replace skill.
Best of both worlds: someone who has passion (and skill and insight) about their task and passion about the mission. The latter can never replace the former. Organizations staffed with sports fans or true believers worry me, because they often use their passion as an excuse for poor performance. What worries me more are the employees who have neither expertise nor passion.
(All that said, I've never met a great marketer who wasn't passionate about what she sold. In the case of marketing, it's not just a nice combination, it's a requirement.)