Durham (and York for that matter) are just far too big geographically compared to, say, Peel or Halton. It's been my impression that rural Durham (Clarington, Scugog, and Brock) feels, both culturally and economically, more Kawarthas/Central Ontario than GTA. As for the suburban southwest of the region, out of the three directions the GTA sprawls out into, the East is the least populated. The entire population of Durham could fit into Mississauga with room to spare. It also suffers from the fact that Durham is, geographically speaking, the least connected GTA region. Remember, once you get into the far East of Scarborough, Lawrence is right by the lake's shore, which, combined with the Rouge Valley, means that few of Toronto's main arterials penetrate into Pickering (compare to Mississauga, Vaughan, Markham, etc.). Additionally, Durham's population is concentrated along the 401/Lakeshore East corridor with few other routes into the city, and little of the North-South depth that characterizes the more connected Western GTA.
Oshawa doesn't really fit into the Toronto suburb mould. It's probably the most self-contained city in the traditional GTA, comparable to Hamilton moreso than, say, Vaughan or Oakville.
I don't really see the point in integrating Durham more into the GTA. Pickering, Ajax, Whitby, and suburban Oshawa are already thoroughly and unmistakeably GTA. They all have all-day GO train service (unlike Brampton, Markham, etc.), and demographically-speaking, are far more similar to other GTA municipalities than to the surrounding rural hinterland. Leave the small towns and farmland alone.