Mini Spool question

The moly pin is optional because it would be an optional replacement for the pin thats already inside your carrier...The pin that the spider gears rotate on.

The pin is how the mini spool attaches to the carrier and is required otherwise your 2 axle shafts would be locked together by the mini spool and the carrier would just spin freely around the shafts and you wouldnt go anywhere.

Id say go with the Moly pin, Im not sure what kind of steel the pin in the carrier is but for 15 buck you cant go too wrong knowing you've got a pin that was designed with the mini spool in mind.