Many users will see the first few words of the welcome. Few will read enough to realise they can verify. Even fewer will actually verify.
Later, your users will wonder why they're not receiving emails from you. If you send them out despite not having verification, then you've got the abuse problem again.
Abused (non-)users will just see every email as spam and ignore them, including the first welcome one. If you continue to send emails out, they will continue to be abused.
