A PC behind the phone could be downloading a file via ftp or network share that could kill a voice call, if voice packets are not prioritized over data. This prioritization and forwarding of traffic is done via qos.
While I understand you're concept, I think that your logic is a bit off. The fact that voice traffic is real-time it becomes sensative to these peaks and valleys of network traffic. I've seen systems thrown out b/c QoS was not configured. On a small Cisco Lan, this can be configured in minutes. I don't think anyone would argue that statistically you may be okay - but with each network different situations arise. Part of the answer is that it is best practice. If you put in a LAN only VOIP environment today, I can almost guarantee that you will soon have phones across a VPN or other WAN link shortly. That traffic has to be marked and prioritized from point to point. Had you taken care of the LAN tagging in the first place you would cut down on your config and potential mistakes on the upgrade. Also think about the next guy behind you. If you ever walk away from your environement to persue greener pastures wouldn't you like to hand over a masterpiece than an unfinished project?
In the old days there was a saying that no one ever got fired for buying blue meaning that if you were questioning what system to buy, buy IBM. The same applies here. No one has ever been fired for applying QoS policies to their VOIP/Video installs - but plenty are gone who have not.
If you get in the habit now of creating a complete config you will thank yourself in the long run.
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