Sir John Carling Building has irreparable structural problems. I was told ten years ago that the columns in the tower were cracked, although I have not seen any real confirmation since that is indeed a fact. Regardless it WILL be torn down.
As for the Lorne, it had structural problems within the foundation itself. This article oversimplifies the actual problem as "slowly sinking into the ground"
http://www.obj.ca/Real-Estate/Constructi...ng-begins-Lorne-Building-redevelopment/1
With existing buildings, the shell can be demolished, leaving the structure for reuse. The problem with that strategy is that sometimes the floor to floor height is insufficient to put in proper ceilings with enough room for all the lights, ductwork, plumbing, and IT stuff that modern buildings require under code and for getting the space leased to selective tenants. Reusing the structure is worth a few LEED points, if the building is going to target that standard as a goal. If done right, the results can be spectacular as in the case of the Toronto BMW dealership downtown.