Jay,

You are of course correct, such profiling will not do as much to relieve stress as kerfing might. But, it has the advantage of being attractive, adding a decorative element, a certain architectural embellishment that reflects the tremendous skill of the carpenters who built these things.
The profiling is more prominent on door and window posts where splitting would be a major issue -distortion of these members would mean the doors and windows attached won't operate properly. The result is both atactive and effective at preventing cracks and twists.

As for the mortising approach,
I have never used a chain mortiser, probably never will. I'd have to fall in line with Tim, and suspect his solution might be best for you. Why, I might even buy it off you when you're done!