Agreed, the Bloor stop is key. It's a small percentage, yes, but for a lot of Line 2 riders, it would be 100% pointless to take the UPX if they had to go all the way to union as opposed to getting off at Dundas West.
Eglinton I have more mixed feelings about. In the beginning I'm fully in support of it for the same reasons as Bloor, i.e. Crosstown riders will want a local connection, not to go all the way to Weston or Bloor (they'd likely just take a bus or a cab instead); HOWEVER once the ECLRT is extended to the airport I can't imagine anybody getting off the LRT to take the UPX at a much higher fare when they could just stay on the LRT and be at the airport almost as quickly (once you factor in transfer time of up to 15min for the UPX, and any walking necessary at that interchange). I'm one of the bigger proponents of the UPX that I know of so far, I absolutely love riding it and use it at least once a month either for the full length or for a fast trip to/from Dundas West, but even I can't imagine myself getting off the LRT for it so close to the airport if the LRT also ends up going that far.