Suppose that Romney was interviewing people for an important job. The person who got the job would get to exercise enormous independent discretion, and his or her character and values would therefore matter a lot.
Now suppose that a job applicant simply flat out refused to let Romney see highly relevant personal and business history information that would have shed considerable light on his or her character and values. Do you think that person would get the job?
UPDATE: As noted by a commenter on this blog entry, it would certainly be interesting to learn how many past years' tax returns Romeny requests from people he is considering for the vice presidential nomination.