As we know, the hole is there to stress relief the end of the circumferential weld. As to the plug, good question but I think its just there to avoid excess oil escaping. The important point is to avoid containing any moisture. The inside of the shaft is ventilated elsewhere (oil for spline lubrication runs out at the bottom) and if the plug is a good fit, there is no cavity to trap moisture. That just leaves the moisture trapped in the sludge, which recurrent cleaning will eliminate.