Have you ever installed the plumbing in a large office? Neither have I. And if I did I would hold out much hope of it not springing a leak or two. It's a complicated business and you would think twice about having your corporate plumbing installed by anyone other than a trained professional. It's a shame that most organisation don't apply the same rigour when it comes to installing P2P processes.
It's a useful analogy. Installing only half of the purchase to pay process is like installing half of the plumbing. Turn the water on and it will be like a monsoon. Both ends of P2P need to be joined up - but knowing which purchasing component joins with which payment component isn't always obvious.