The GTA already has several business districts. In fact, downtown Toronto makes up a very small percentage of jobs in the region are located there. However, the GTA does lack a true multiple central business districts. Areas where people can live, work, and play in a walkable environment. These areas should also have enough to attract people from all over the region to visit as well, preferably by transit if possible.
The closest to such an area would be downtown North York. It is dense; good mix of residential, employment, and commercial; and has some pretty strong transit connections as well. When Jersey Boys was playing at the Toronto Centre for the Arts, it also gave people a reason to go there. Still, it needs/ed more to get people to go there, a good and unique variety of points of interest. While still striving for the urban environment that downtown North York has, downtown Mississauga has Square One, Playdium, and the Living Arts Centre to get people to come and visit.
To the topic, yes I do think we need to take the idea of multiple CBDs seriously. While quiet suburbia isn't necessarily good, neither is overcrowding. In a city with the population and area so large, we shouldn't put all our eggs in one basket and provide true CBDs on a more local scale.