As for the reason why shomen uchi is learned historically? Much simpler answer: sword cuts are done in the exact same motion, and when trying to evade that, you should look at the hand instead of the blade. The hand will tell you where the strike will come, and evading that and countering is exactly the same, with or without blade. The important thing is to understand line of attack, timing, and proper footwork. Everythings will come through following that.
Not a bad answer IMO overall.
Simply put, for me, shomen strikes give us a very simplistic, controlled attack that filters out all the other variables that allow us to explore the basics of irimi and ma ai. It narrows things down to a set of parameters to encourage the development of proper habits of movement and posture.
It has nothing to do with tactics or application at all.
Steve Seagal is no more lethal with his shomen strike than a six year old. It is simply a train methodology, and one IMO that works pretty darn well for what it was meant to convey. Don't read into it too much IMO.