Out with the network card, out with the SCSI controller. Then again, without a SCSI controller, the system won't boot, either. In a multivendor network, no vendor can be held accountable. Furthermore, if a problem crops up, the culprit is likely to blame the competitor's equipment. Single-vendor networks shift the burden of accountability.
Accountability is by no means a golden passport, especially in today's acquisition-oriented market.
However, at least when a problem does crop up, it's you who can point the finger. Maintaining Employee Sanity Count equipment support and maintenance as another big plus in buying from a single vendor. Your limited IT staff is responsible for all the equipment on and attached to your network. These staff members must understand, install, service and live with it. The more product families deployed, the less your IT personnel will understand each platform.