There is significant benefit to OW from an alliance-wide link with B6. Look at B6's major markets:
JFK -- the premier East Coast gateway; almost every OW airline is here
BOS -- BA & IB serve BOS, and would benefit from B6 feed
IAD -- connections to big leisure destinations CUN, FLL & MCO
MCO -- served by BA & MX
SFO -- incremental additional feed for JL & CX to NRT/HKG (esp. to AUS?)
More importantly, in the Northeast, B6 feeding AA/OW at JFK would be at the expense of US feeding *A and DL's feed for itself/ST.
Maybe this is a sign that OW is finally getting serious about acting for each others' benefit?