Today's competing non-SCSI solutions lack the features and connectivity needed to provide a comprehensive I/O interface solution. SCSI is the only high-performance interface that allows devices operating at different speeds to be connected to the same bus, and the only I/O solution that offers a method for connecting both internal and external SCSI devices. UDMA/IDE only supports two drives per channel, thus can only handle a small range of internal devices.
Furthermore, although UDMA/IDE has come a long way since its initial introduction, all mission critical storage subsystems use SCSI, as its MTBF rating, the time a given product of a given design can last without requiring repair or replacement, is much higher. As a result, all high-end HDs use the SCSI interface.
Fig 1. The Ultra160 68-pin SCSI connector as featured on the Quantum Atlas 10K II.
Also, a SCSI-based system doesn't impose much overhead on the main CPU, all data I/O and the controlling and handling of the SCSI devices are done primarily by the SCSI adapter, freeing up the CPU for other tasks such as fetching data and running applications.
Overall, while lower cost makes UDMA/IDE a viable solution for less demanding computer environments like basic desktop PCs, it lacks the power and the broad range of connectivity features needed by workstation or server configurations.