Well, your supposed to do a visual inspection for corrosion and dirt(from clutch wear). It's only a few extra minutes to do correctly.
The diff covers I'm adding will have fill plugs on them. But I have no interest in a drain hole. It would just add a weak spot. Don't forget how many rocks and logs your diffs plow into. I suppose a drain on the cover is fine, but not on my cast iron (soft) axle housing.
Beware diff covers that have the fill hole way too high. They're usually the same covers with drains like this pos:
I'd rather have this, and waste some spill pads: