Are the benefits that significant? After all, boarding times are significantly slowed down by people trying to cram suitcases that ought to be checked into overhead bins. Also, quite a bit of carry-on luggage ends up being checked anyways once the overhead bins fill up. I would love to see some real analysis on this.
The problem with checked luggage first arises if somebody doesn't get onto the plane in time even though their luggage has already been loaded. This essentially means that somebody has to look through all the (already loaded) luggage to remove that specific piece, which, depending on how deeply it's buried, might take considerable amounts of time. This isn't a problem with hold luggage, since there cannot be any hold luggage aboard a plane without a corresponding person.
It still applies to international flights (presumably for customs reasons).
Also consider that usually ground handling is a different company, the airline is paying the stewards wasting time on board anyway, but may have a per-bag price of the check ins.