Personally I think it's ok to take stuff if the building has no reasonable hope at being rehabbed and nothing you take damages the building during its removal.
This is my philosophy too. Several of the things I've taken home with me had already been removed and discarded by vandals or scrappers, or were the kind of stuff that would just get binned when a cleaning or demolition crew came through, depending on the ultimate fate of the building.
An old can opener from a break room, or 70s soda bottles, stuff like that is really neat to me and probably a lot of you, but it's just trash to be thrown away or shit to be broken for fun for others. I don't feel bad about taking such things and displaying them or keeping them as a memento of an adventure. I personally draw the line at taking things for the purpose of reselling them though.