I worked with some supply chain consultants who mentioned "internal suppliers are often worse suppliers than external".

Their point was that service levels are often not as stringently tracked, SLA's become internal money shuffling, but the company as a whole paid the price in lower output/profit. The internal partner being the default allows an amount of complacency, and if you shopped around for a comparable level of service to what's being provided, you can often find it for a better price.