Oh yeah, that's due to a change on 2.0. Now the cache lives on the engine's ram and is reloaded through the unix socket. So what happens when you try to use rebuild when booting is that it won't have a socket to connect to.
At the same time, you don't need to rebuild most pages when you reboot, because all the cache was trashed when the engine stopped. Only files that are uploaded to mongo are kept through reboots, like the default thumbs and full static pages, like the 404 and maintenance.