I've said this for ages!
Salford and Trafford contribute to Manchester's GDP. Without them Manchester wouldn't be anywhere near the size of city is it viewed upon as by the rest if the world. Europe's largest industrial estate, Trafford Park; the Trafford Centre; Manchester United (boo!); Salford Quays, Trafford Wharf, MediaCityUK; and many other entities all contribute to Manchester's GDP.
Everybody's city centre, or "town", in Salford and Trafford (and much of Tameside) is Manchester city centre.
I think the city centre should be pushing to be a bit like Berlin city centre with it's long stretched out look. From Salford Quays and Trafford Wharf in the West, through Odsal Islington and Castlefield, to the city centre as it is now, through Ancoats MilesPlatting and New Islington, to the Etihad Campus and it's surrounding areas in the East. There should be an aim to make that whole stretch our city centre.
Manchester has never been just the approx.500000 citizen borough. That's all that is, the Metropolitan Borough of Manchester. The city region of Manchester is far bigger than that. Manchester used to be a little district in Salfordshire/the Salford Hundred. But Manchester grew. It engulfed other districts that were once just part of the Hundred. Now, almost exactly as London did and sort of similarly to how Stoke-on-Trent did, I think it has engulfed boroughs towns and another city to make Manchester a true city. Salford and Trafford are part of that, as are parts [but certainly only a few parts, a small percentage] of Tameside Bury Rochdale Oldham Stockport and North Cheshire.
To keep getting investment we need to grow. We need to nudge ourselves ahead of Birmingham and Glasgow, otherwise we're just going to be competing with those average cities, like we are now, staying an average city ourselves. This is one part of a perfect opportunity we've got to make us the true second city of the UK and really push us up the Global City Index. Which would entice further investment.