I think therapists with insecure attachment styles *can* help clients with attachment issues, but they must be aware of their own attachment style and understand how their attachment style influences the therapeutic relationship. There's a really great book called Attachment in Psychotherapy that touches upon these issues. (I would link to it, but I don't have permission to link to things yet.)

50% (or more) of therapists are simply incompetent. It has nothing to do with their attachment styles.