Hudson, training may be expensive but it is never, I repeat never superfluous.
I'm sure you and I have both flown with F/Os who were near perfect and 99.9% will transition easily to the left hand seat without a hiccup but that still leaves a small number that suddenly discover that when all the responsibility rests on their shoulders their ability to make rational decisions, fly the aircraft and command a crew goes out the window. Surely far better to discover this in a sim, (and be able to correct it with further training), than assume that previous line flying experience as an F/O is all that is needed to assume the role of commander and then be proved wrong?
As an F/O one may show all the attributes of a captain but until the total responsibility comes to rest on their shoulders they have not been completely and properly tested. Even in operators with the most advanced training systems there will always, unfortunately, be a few command course failures.