FCP X can directly upload to Youtube from the share menu. I've heard mixed results on using this option, it could be different now. Encoding directly from FCP X utilizes the GPU which can speed up encoding. I could see using the export to Compressor option if you encode to several different formats, to utilize multiple cores of your CPU or use multiple macs to help render the video on a network.
I would do some testing on several ways to see what is better/faster for you.
Here is a few links that might be useful for you that are easy to understand on using Compressor 4.1:
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