Hi all!
Personally, I have a theory regarding the origin of the shardblades/plates, that is hinted by the chapter where Dalinar uses them as a work tool.
So, what if they WERE work tools someday? Or, at least, purely nonlethal weapons. Perhaps not shardblades themselves, the the principle behind it, save a few late tweaks (see below).
I mean, look: an omnicutting weapon that does NOT cut living flesh. Once the flesh is dead, however, it cuts it easily. It can only be INTENTIONAL effect - otherwise, in the language of TV tropes, it is "Awesome, but impractical" - simply cutting flesh as it cuts anything else will produce comparable or even better effect - it is noted several times battling with a shardblade that does not sever as it kills can sometimes be detrimental. Otherwise it is just 'for coolness sake', and I truly hope that Sanderson is above that.
Perhaps, given the large doze of divine interference in the story, they were originally created as either 100% safe work tools, or weapon with zero lethality but to nonliving (inorganic/inhuman) opponents.
But then they were somehow corrupted or tweaked that they began to 'cut souls'.
It would explain why Syl is so disgusted with shardblades - they are not 'mere powerful weapons', they are perversions of something that once was benigh and perhaps holy.