Now, I'm not expert in Aikido, and I doubt I ever will be, but I've done quite a bit of research and reading myself, and I just wanted to comment on one thing, if I may.
The shomen uchi is not, I repeat, not an attack. It is by no means an proper atemi or something you'd expect on the sidewalks or battlefields. What it is is a teaching aid, a way to simulate an attack.
You see, there's only a few ways to attack with arms. Whatever shape the hand is in, the arm is restricted in it's motions due to human physiology. Aikido is a studying of human movement afterall, so you'd expect something like this to dictate attacks.
For strikes that do not grab onto opponent, which end in a connection point, we classify them into: shomen uchi, yokomen uchi, chudan zuki, jodan zuki, gedan zuki, and various others that I don't know yet. For shomen uchi, it is the strike downwards from the top, down your centreline, and cut through, just like a vertical sword cut.
But for strikes like shomen uchi, jodan zuki, chudan zuki, and gedan zuki, the strikes all flow down the centreline, which means they move in a straight line towards you. This is the centreline theory of attack, and you see this famously in Wing Chun. For Aikido, the centreline attacks are to be dealt with using evasion, blocking, or flowing with the strike. The reason shomen uchi is used is do simulate an attack down centreline, so that nage (or shite) gets a good feel for the centreline attacks, so that when transition over to jodan zuki, it because easier to learn. The same applies to other attacks that flow down the centre. The idea isn't to master defense against the shomen uchi, nor to learn shomen uchi so that you may strike with it, but rather to learn and understand the centreline theory of attacks.
Now, you can use shomen uchi to attack, but why use it when there's much better methods of striking? Granted, masters like Seagal have perfected theirs into a deadly weapon, almost like a sword, but for most of us, a uraken or shomen zuki would work far better.
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.