SEA_Tigger reported in this
thread , UA makes about $300 million from US codeshares annually. UA had a reported loss for the 2004 of about $617 million. UA can't afford to loose an extra $300 million in revenue on top of their current losses.
Currently UA appears to be focusing more on their international routes then domestic. If that is the case they will probably want the code share even more to help drive more traffic to those routes.
There was an interesting disussion on US and UA and * over
here in the HP board a couple weeks ago.