This was pretty common before the financial crisis, though the typical contract length was a year back then. The idea is that it was a 2 way street- the consultant is committing to your company as much as you are committing to them. And if you are a mega corp, honestly one person's salary for a year isn't really that big of a deal. The cost of hiring and firing a FTE is not much less.

Though the length of it is pretty head scratching. The whole point of using consultants was that you needed a short term amount of specific work done, unless you are talking about engaging large consulting companies to deliver a project.