Actually, these are really interesting problems. Multi-homing isn't an issue for most startups though (not that this is true multi-homing since their second DC is read-only, meaning that they can't really survive the loss of the primary).
But due to the replication delay, that would mean losing data on the primary, and a long delay on bringing the old primary back in sync with the new (since they have effectively forked). Therefore they would only switch the the secondary if the failure at the primary site was catastrophic -- it's more like a backup.
