Another benefit of using contractors in these sort of roles is the fact that they can remain pretty much politically agnostic.

They don't need to worry about sucking up to people to get their bonus, or saying things just because it is what their boss wants to hear.

This may well result in a solution which is architected for the right reasons, rather one that is politically motivated and doomed to failure.

These political motivations can apply equally to permie and consultants. As a consultant, how many times have you been pushed to do something in a particular way by a sales person for example?