The thing is if OP is currently scheduled to fly through SFO to EWR and cannot change the flight then per the UA T&C OP should book another flight from EWR back to SFO. That seems like an awful waste of time.
Let's say there is a 10% chance of OP getting re-routed and skipping SFO (I'd say it is much less than that, but let's just go with 10%). So 90% chance of saving basically a whole day (SFO to EWR and reverse). Not sure how much that is worth to OP, but to me that is worth a whole lot, particularly given it is just pain and suffering in economy.
And 10% chance of having to book another ticket from EWR to SFO at the last minute (a day in advance, given flying from Asia with connection). Let's say it is $400, and on OP's dime.
So is it worth $40 (10% of $400) to save 10 hours (90% of 12 hours)?