Except Metrolinx was stupid enough to have a stop at Weston for all those business people and tourists who come from all corners of the earth to take in lovely and exciting Weston. What a stupid thing to do, as if anybody from Weston is going to pay $25+ to get either downtown or to Pearson when it's only a 20 minute bus ride away.
As it exists now, Weston is pretty poor (but fortunately not very violent). I don't think too many current residents are going to directly benefit from the service itself, but the economic spinoffs it generates.
I believe you're correct in that no one will take the UP Express downtown, they will take the GO instead (currently $4.82 w/ Presto). The added bonus with the whole Georgetown South project is you get those GO trains more frequently in both directions.
As for taking the bus to the airport, it's kinda sketchy rolling through Dixon on a TTC bus. I took the Lawrence bus from Weston to the airport a while ago, and while I might be a bad person letting stereotypes prevail, it was fairly intimidating. If I had a choice, I'd have taken the UP Express. I also don't believe it will cost $25 from Weston, I believe Metrolinx will set fare zones like the GO. I'd shell out $10 to get a non-stop comfortable ride to Terminal 1, and have room to place my luggage. I'd guess it would take less than 10 minutes.
Taking all this in, it leads me to these conclusions: a) the ridership at Weston for the UP Express is going to be determined by how much new residential development tailored to frequent flyers occurs here, and b) if a significant amount of that new development occurs, there is a real risk for gentrification on the west side of the tracks.
Maybe Metrolinx's plan all along with the Weston stop was to spur urban renewal. But if so, they need to step up effort with the city to brace for the displacement of poorer tenants here and have a social housing strategy.