Do we need test data or can we rely on unit tests and manual testing?

Question

We're currently working on a medium/large PHP/MySQL project. We're doing unit testing with PHPUnit & QUnit and we have two full time testers that are manually testing the application. Our test (mock) data is currently created with SQL scripts.

We have problem with maintaining scripts for test data. The business logic is pretty complex and one "simple" change in the test data often produces several bugs in the application (which are not real bugs, just the product of invalid data). This has become big burden to the whole team because we are constantly creating and changing tables.

I don't really see the point of maintaining the test data in the scripts because everything can be manually added in the application in about 5 minutes with the UI. Our PM disagrees and says that having project that we can’t deploy with test data is a bad practice.

Should we abandon maintenance of the scripts with test data and just let the testers to test the application without data? What’s the best practice?