WHEN COMPENSATION FOR
THE SKILLS NEEDED ARE EXCESSIVE

Hiring specialized knowledge in IT is very expensive. Years ago this was the case with SAP and mainframe COBOL during the Y2K frenzy. Experts were commanding salaries for permanent positions well beyond $175K. Today experts with genuine skills in Sharepoint and Business Objects are in the same position. They have the ability (and the knowledge) that they can command salaries well beyond contemporaries with more experience but who lack their specialized knowledge.

Why do you need to own and maintain the skills? What problems are being created internally?

Using a consultant allows you to lease the skills for only the time you need those skills. Use a consultant to complete the work and teach your internal team at the same time.