Originally Posted by SixAlpha
My understanding is that it is for ease of booking. Airlines don't generally give access to each other's computer systems, so code-sharing allows one airline to book you all the way through.
The airlines still need the same level of access to each others' computer systems with code sharing. Otherwise, they will not be able to tell each other how full the flight is (and whether additional reservations are allowed and at what price). Not having enough access could mean that a flight is overbooked at the operating airline, while the code share partner is still selling discount-web-special-super-cheap seats on that flight (causing it to become even more overbooked).
Indeed, changing to "honest" branding does not require changing anything other than what is displayed on reservation systems and web sites. Instead of, for example, UA xxxx operated by AC for a code share flight on UA's web site, AC xxx could be shown on UA's web site when a list of flights is shown. They could keep whatever back end stuff goes on the same as it is now.