More useful for connectivity concerns sure. Spadina commuters transferring to the Yonge line via Sheppard West extension would increase the subway's ridership numbers too.
But will Sheppard ever attract a substantial number of new riders, ever? I think not.
There are very few destinations along the Sheppard corridor, employment is not growing, feeder routes go to Yonge subway or Line 2 rather than to Sheppard (a trend that would continue despite a Sheppard East extension) and Sheppard's existent riders are already transferring from the Sheppard bus. A Sheppard East extension is only pushing the transferring point for them further east.
Yes, Sheppard would attract a substantial amount of new riders because the land use/zoning in several areas hasn't been put to its full potential yet. All the industrial lands in-between Agincourt and the Town Centre can be redeveloped. There's patches of vacant land elsewhere along the route, particularly around Warden and Victoria Park, that can be rezoned and developed as well.
And now onto the major connecting surface routes. 24/224 Victoria Park, 68 Warden, 17 Birchmount, 43 Kennedy and 57 Midland in total see a daily weekday ridership of 85,000. The 190 sees 10,000. So it's safe to say a good chunk of those passengers would transfer at a Sheppard subway stop rather than continue south on the bus to Eglinton or the BD. Does a Sheppard subway really sound so unreasonable, then?
Since Sheppard West has to be built as a subway to Downsview (no room on the surface for tracks), the case for just restoring the entire Network 2011 plan altogether from Dufferin to McCowan makes so much more sense. I don't think we ought to delay, certainly not til mid-century. Construction on several corridors should be ongoing. Being snobbish or prejudice against extending the subway, for whatever reason, fixes nothing.