You know what, I think hes right. Yes the defending team will have to place some sentries places, etc, but the sole objective of the attacking team is to all be in one place. The defenders will be fireing rockets and stickies into a croud of people, getting random kills with no skill.... and i also see absolutely no purpose for the Spy as a defence... or Scout...
The scout will probably increase the speed of the bomb, so you can get a scout to the front lines quicker than anyone else. And a spy could be used to make sure they don't have chance to pop an uber.
From the GameVideos video I posted above as well, that map looks like it has lots of areas to sneak up around the bomb or cut it off, things like that. So you'll have to make sure you're covering all areas. Rather than just standing in one spot waiting to die.