It adds one throat to choke. You won't have separate vendors for the OS and the hardware to point fingers at each other. In the case of the mainframe, there's u

It adds one throat to choke. You won't have separate vendors for the OS and the hardware to point fingers at each other. In the case of the mainframe, there's usually a whole raft of other software that's all from IBM to add to the one throat to choke: TPF, z/OS, z/VM, CICS, DB2, etc.