Complete system tests will uncover the same problems as complete unit tests.
In fact system tests provide more customer value than unit tests because
only the functionality being used by the customer is tested. Exhaustive unit
tests do not provide customer value. Test based on customer usage provide
Yes, but without unit tests it's harder to keep the system tests working. (by system tests I'm assuming you mean acceptance tests) -- DavidAndersen?
Tests don't provide value at all. Features provide value. Tests are used for several reasons: