Tor is simply an additional layer of obfuscation. Assuming tor is completely compromised by LEAs, it wouldn't matter because nothing being passed over tor can be used to deanonymize the user. Before being sent, all messages are pgp encrypted before being encrypted again by BitMessage. BM doesn't send any user information in the messages anyway. No one can discern who sent what and for who. As I understand it I2p support will be added later.
Feds haven't been able to prosecute anyone who used BitMessage for perpetrating a crime. BM is battle tested.