I think for a full run to and from Dulles and Ashburn, an express train has diminishing utility but the time savings within that express service bracket like Tysons and Ballston and close to the skipped stations might see significantly time savings. If a person lived in Ballston, they could get to Rosslyn, Tysons or Reston much more quickly and because the distance is pretty short I think they would be less likely to 'catch up' to another train.

I know that's not why they are implementing it am just making the observation who the major beneficiaries of this change would be.

I'm somewhat confused why they would operate express-train service where the incremental operating cost exceeds the incremental revenue.