I just read on another thread (just a few minutes ago, in fact) that folks cross breed these two for brooding purposes.
Bantam silkies make great brooders for folks who do not want to rely on electric incubators for hatching eggs.
But they are small, and can only sit on a few full size eggs at a time.
By cross breeding a bantam silkie with an ameraucana roo, the resulting pullets are larger than their bantam mother. They still seem to like to go broody, but when they do, they can sit on more eggs than the bantam mother can.
I don't know anything more about this than what I just wrote. And the only reason I know this much is because somebody was talking about this on another thread.