There are several things that I did not see in the article. It seems to imply that Voip handsets and Voip servers share the same infrastructure as workstations. This is already recipe for trouble as even a normal business use could seriously affect the phones.

Today, these networks really need to be separate, either physically or through VLANs, as a lot of network architectures cannot readily address both needs: throughput for business application or latency for Voip.

The only application of NAC that I can see useful today is if somebody mistakenly (or not) connects a PC to the Voip network. Using NAC (802.1x actually in this case) would make sure that this device cannot connect at all and that the disruption is limited to that port.