When you use something like Electrum to generate a brain wallet, the passphrase is the seed. From this seed, the app generates an unlimited number of addresses, public keys, and private keys that can be deterministically re-generated, given the passphrase. So in this case, you can have one "brain wallet" (the passphrase) but an unlimited number of public keys generated from that seed; and revealing any of these public keys should be perfectly safe. Right?