I keep running into this and I thought it would be an interesting question. Quality of Service is a pretty common network configuration to help with ensuring your voice quality on enterprise networks. I have come across a number of large enterprises (10,000 + endpoints) that have deployed IP telephony and haven’t deployed QoS. They have gone for the approach of keeping the network simple and making sure there is adequate bandwidth.
The benefits of QoS are of course well documented and not just limited to voice. It certainly brings forward a number of points about cost of network to complete QoS over bandwidth upgrades and to what level is voice and video acceptable. Of course the world’s largest network (the Internet) doesn’t support QoS with millions of people using it for voice, video and everything else every day. So what side of the argument do you stand?