It's also very important to note the difference between a building, like the Bastion, and a ruin, like the CoD terrain. The Cities of Death Rappelling Lines are for getting into the upper levels of ruins (CoD, P. 38). If there's already someone in the Bastion, as per the building rules in the 40K rulebook, (p. 79), you have to actually attack the building- you can't walk into the building as it's already being held. Keep in mind that buildings are treated as immobile transports. Would you try to land troops on top of someone's Rhino? The Scouts being on the roof of the building make it count as Open-Topped (40k rulebook p. 80), it does not make the Scouts legal targets for shooting or assault.
The moral of the story? Buildings are really, really good for units with large potential shooting output that suffer badly in close combat. Also, it is important to look over the table pre-match and double check that everyone understands the effects of every piece of terrain. If I had a dollar for every argument prevented by that opening conversation, I'd probably have another Company of Ultramarines.