A graphic example of the problem:

As can be seen from the images above it would be impossible to fit a CFL field into BMO Field without significant rebuilding of the north and south end. Some have suggested that a new stand or sections of a new stand could be built on rails thus allowing it to be repositioned for either soccer or football but this ignores one of the central concerns of Canadian soccer supporters: BMO Field is a purpose built soccer stadium intended to serve as a home of both Toronto FC and the Canadian National Team. Rebuilding the stadium so that it becomes a multi-sport arena is a fundamental betrayal of the attempt to create a new permanent home for Canadian soccer. Further, in an era when more and more Major League Baseball teams and NFL Football Teams are discovering the benefits of moving out of 70's era multi-purpose arenas for purpose built single-sport stadiums it is beyond foolish to presume that there would ultimately be a comfortable way to accommodate two sports in BMO Field. Ironically, it could be reasonably argued that the Rogers Centre, the current home of the Argonauts, was the last major North American stadium built with the multi-sport philosophy. Consequently it suffers from many of the problems inherent in that era of stadium design: lacking the intimacy of the best baseball stadiums while being too small for NFL football and too big for CFL football. As the recent abandonment and demolition of the oft derided Kingdome in Seattle, Riverfront Stadium in Cincinnatti, and Three Rivers Stadium in Pittsburgh illustrate when compared to the continuing fan affection for such classics as Fenway Park and Wrigley Field it is often far better to design a unique stadium for a particular sport rather than attempt a one-size-fits-all hodge-podge approach.