Foreign Key Importance

We don't have foreign keys (FK) in our database.
I think FK's are very important to enforce data integrity and also it's very easy to understand data flow. But one of our developer argues we can maintain data integrity through java code, without FK's it's easy to merge/migrate data (TRUNCATE works without FK's).
My point is we should fix migration script otherwise we end up several with several orphan records(we already have 25-30% orphan records), also some time there could be bugs in Java code. This also complicates simple database to impossible to understand especially for new team members. Can any data guru advice me what are other benefits of FK's (coding time, performance, indexes...etc).
Thanks.

foreign keys must and should be there.
only a novice user will suggest not user those not a real developer.