You both have very valid points; traffic on Brock Street (and Brock Road, further west at the current end of the 407 in Pickering) can be brutal at times. I suppose my primary objection is to the freeway nature of the West Durham Link (esp. next to or directly through the narrow strip of the Greenbelt between Ajax and Whitby) ; I don't see it as being a necessary configuration. Lake Ridge is an 80 km/h road with very few intersections. Admittedly it is only two-lanes but for the price of a brand-new highway and complex freeway-to-freeway interchanges a fair degree of upgrades (widening to two lanes, full interchange with the 401, even grade separation at busier intersections if warranted) could be made to turn the existing road into one serving the same function as the WDL instead.
At the end of the day I also agree about the necessity of the 407 East as a whole, especially the revenue that its tolls will place into the provincial coffers (where, had it not been for Harris, they would/should have been going all along).