SHARES is vastly inferior to UA's airport-side IT infrastructure.

Rebooking and reissuing a ticket is super-easy in UA's system, CO's is a nightmare. As channa said, ease-of-use affects the willingness of agents to re-issue tickets in IRROPS. It takes a CO agent 10 minutes to do what takes a UA agent 20 seconds. If they can port the UA interface to CO's backend I suppose it would work fine.

Also, doing VDBs is a piece of cake in UA's system, CO takes a lot longer. I guess in that situation they gotta do it either way but in a situation where they need to get the flight out and there is a rebookable flight leaving soon it can make a difference.