I don't necessarily disagree, but two other points to consider:

1. Every test that is written is another use case that wasn't tested before. 100% test coverage is often impractical, but the more tests you have the more of the code you can be confident about.

2. Every test you add is another regression that can't happen in the future; if you test the index rebuilding code and validate the output then you know that you aren't going to make a change that breaks the index rebuilding code. If you have a legitimate change you update the tests, but if you're not expecting the change then you know there's a bug somewhere.