By using a USB mic, you eliminate the need for the mixer, but you also limit yourself to the one track you are using to record, as well as the A/D converter quality that is provided with the mic. In the setup you've described, the mixer isn't doing anything for you.
Pete's setup allows him to track a guitar and vocal track simultaneously (two microphones) as well as upgrade his A/D interface if he wants.
I don't prefer USB mics for exactly these reasons.
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