The lack of employment lands in the CBD is a legitimate concern. However, I think it is prudent to always build residential and office/employment/institutional uses in the same area. It allows for a more vibrant neighbourhood at all hours of the day. And if you don't buy that argument, the real argument is one of infrastructure. The city pays X hundreds of dollars for roads, sewers, pipes, etc for every parcel of land. I'd rather see those dollars be used all day and all night than selectively so that one area can be residential and one area and be solely office.
As has been stated already, there really is no shortage of land available to expand the CBD or build up other nodes in the city (Y&E, Bloor-Yorkville, NYCC, Bloor W, Portlands, SCC, Sheppard corridor, etc). They key is a mix of uses, intensification, in-fill and beautification.