I'm thinking more on the lines of how the western half of the city would now be more accessible to those in the Yonge line and the eastern half of the city more accessible to those on the Spadina line.
Some Yonge line users will transfer on Sheppard and again on Spadina to go downtown if it will reach their destination quicker/with less congestion, and vice versa for Spadina line users. (Come to think of it, this would help reduce Yonge-Bloor station capacity issues) Other trip generators besides downtown, such as Fairview, Yorkdale, NYCC and York U would now be much more accessible by public transit to a much larger portion of the city. Besides that, the subway would absorb the Sheppard West bus ridership, how ever many passengers that is.
We will have to wait for an EA to see exactly how much ridership we are talking about here, but something needs to be said about the benefits of connectivity in the transit system.