If you have a set of unit tests for a large application, and not every one of them passes, then you don't know whether the application works correctly. In fact, failing unit tests are evidence that it isn't working.
Also, marking them with the [Ignore] attribute doesn't count as fixing them.
Also, deleting the part of the unit test that fails doesn't count either. An empty test method is not a passing test unless the method being tested is also empty. And an empty test method should not ever be committed to the source repository.