This isn't simply the "one throat to choke" mentality (although that's certainly part of it). Single-sourcing one's manufacturing needs is smart. Done properly, it simplifies the procurement process and reduces product lead-times. It strengthens the customer-supplier relationship. Engineering teams on each side of the fence are more likely to collaborate, increasing the likelihood of product improvements and cost reductions all around.
The problem, though, has long been one of capacity and skill. Machine shops don't generally "get" sheet metal, nor do they have press brakes or turret punches on the production floor (although an increasing number are turning to waterjet machines and electrical discharge machining [EDM]). Virtually all fabrication houses, on the other hand, have at least some machining capabilities, needed for tool and die work — asking them to spin up some special screws or mill a few housings is no big deal. The typical CNC machine shop, though? It's unlikely to supply any sheet metal parts without subcontracting them to the fab shop down the street.
The moral of the story is simple: The most successful shops are (or soon will be) the ones that embrace all types of metalworking (as well as supporting processes such as plating, painting, powder-coating, and assembly). They have learned to cross the party line and become masters of both chip and chip-less manufacturing, including three-dimensional printing, which will continue its rapid growth as an end-use parts production technology. This means future metalworking companies will offer bending, forming, stamping, and machining capabilities, assuring cost-effective and on-time delivery of quality products to whatever customer is asking for them.