One of the drawback is that crews don't always enter the correct callsign. <-- the same could be said for entering the correct SSR code, both are easily changeable as far as I'm aware.
My understanding is that ORCAM codes are still issued because not every ATCU uses mode-s as the primary means of identification. It also helps if an aircraft is squawking a discrete unit code a service is being provided outside of controlled airspace as the controller knows which unit to get traffic information/coordination from.