Arrangers’ extra outputs are all without effects, and drum kits are pre-panned in stereo. Some arrangers might allow you to edit the kit to pan all sounds to one side, but you’ll have to edit ALL your registrations to do this. And the reverb return isn’t (afaik) ever pannable, so reverb from sounds panned into one channel will still come out of both main outputs.

I’m a little confused… what’s the advantage to mixing the Parts externally rather than internally?

All in all, modern keyboards aren’t designed well for separating parts from each other when the effects and inserts are hardwired to the main outputs…