The overall point
there are duplications of effort across corporate structures...on of those duplications is management staff. If an EDS has IT operations for several dozen corporations, they can leverage than management across all of the clients..effectively reducing the overhead and thus are able to pass savings to clients.
They also benefit from scale to procure resources...or do you think your company can match consummables pricing acheived by EDS or IBM?
There are as many benefits as their are drawbacks in number, the culture often determines the decision to outsource or not.
And if an internal staff is more effective and more efficient, (ie well run) then the business case for outsourcing the function should fail every time...but it doesn't.
So, for some orgs it can both save money and increase efficiency. For others it won't.
There is no "right" answer here.
If you push something hard enough, it will fall over. Fudd's First Law of Opposition
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