This is heavily based on an older thread about the decryption of Navani's sketches, found here:
http://www.timewastersguide.com/forum/index.php?topic=7882.0Great thread, by the way. In any case, that thread gives a lot of information about fabrials, including that they are powered by stormlight, and that they contain trapped spren. That lead to the question if maybe shardplate is a form of fabrial - including a trapped spren. It does regrow, so it definitely is special.
But shardplate is still ordinary compared to shardblades, which can appear and disappear into mist. Also, Syl does not like shardblades at all. But if fabrials contain trapped spren, what could be worse than that? Some people discussed shardplates also containing a trapped spren, but maybe that's still not far enough?
What if a shardblade is a type of spren?
Their appearing and disappearing, their different effect on living and dead matter show that they are very unique. There are some inanimate spren I think (one type was described as being like snow flakes). The Radiants had shardblades, but maybe they got these spren with the acceptance of the spren (like Kaladin acquired Syl). What if the remaining shardblades are sort of enslaved (or "lobotomized" in some way)?That could be why just anyone can use them. That would then definitely be revolting to Syl. And since there are so many different types of spren, it could well be that the blades are among them. That would also explain why the spren-powered fabrial of the half-shards can stop them...
Plate may be similar, but the shardplates of old exhibit different properties - the helmets of the Knights Radiant from the visions appear and reappear like they are being summoned and released just like shardblades. Modern shardplate does not do that - maybe the spren is dead, which would make it less revolting. The plate still regrows though, so there is something there... Syl never said that shardplate is ok, though - maybe Dalinar was just half as bad as before when he gave up the blade, which from her point of view would still be better than both plate and blade. Since the only "feels" this, she doesn't really know what's going on, either.
Anyone have some input on this theory?