The 'custom' explanation is what I had heard before, thanks for finding that.
There is not much material removed from the side gears for the c-clips, I don't think there is a strength issue with those. If you trash your spider gears, it's probably not due to the slight lack of material in that area.
All the busted D35's I've seen have snapped an axle shaft where it first necks down, before the splines and long before the c-clip area of the shaft. During twisting force, this is the weakest area, where it necks down.