It could bring in extra revenue from more connecting passengers abroad, such as passengers from KE and AF/KLM from ST or JAL, CX and BA from OW. It'd be plus for either alliance as it will allow more freedom for major intra-West Coast routes like YVR, SEA, PDX, and LAX which are lacking on both parts of each alliances' North American member, DL and AA. It may even provide an incentive for an alliance customer abroad to choose AS over say, Virgin America, JetBlue or Westjet. In this sense, it might make business sense for AS to choose an alliance instead of sitting somewhere in the middle between OW and ST. Why should a JAL customer abroad take AS when he/she wants to go from LAX to SEA, when he/she can find a cheaper fare online by booking through Virgin America? Why should a KE customer in SEA fly AS to BOS when they can find a cheaper fare on JetBlue?
AS gets most of that today. Look at an AS flight sometime on a flight board at LAX. You'll see it as AS XXX, AA XXXX, QF XXXX, BA XXXX, KL XXXX, DL XXXX, AF XXXX, KE XXXX. AS codeshares very evenly between ST and OW, much to their mutual benefit.
One of the problems with AS picking an alliance might be losing the money from codeshares with the other alliance (if those lapse) - and that's a MUCH more significant downside than MVPGs getting lounge access and RTW awards. AS's route network works in a world where a huge percentage of it is exposed to WN in no small part because they derive lots of codeshare revenue. Losing portions of it via takeover or by joining an alliance would be a serious problem