Vendor: Do we need Quality of Service with shared storage arrays?

Awesome network or not, we need to look at the bigger picture

The Storage Architect I recently had a discussion with a vendor (who shall remain nameless) as to whether we really needed Quality of Service (QoS) in shared storage arrays. His thinking went as follows: if we have a storage array and network with sufficient bandwidth/IOPS, then why bother implementing QoS?"