Now that I think about it, the worst code seem to lead to situations where the top developers can really show their value.

I mean if the code base is quite good and is kept that way, then a normal developer can deal with it quite well. But if you get a legacy mess, then the hard-core developer can make really drastic changes that have enormous value to the maintainability, future development and bug-wise.

If a normal developer tries to tackle that mess, it's easy to get into trouble and drown in the sea of opportunities and not see the big picture and take a proper road plan to improve things.

So in a way -1 developers enable the top developers to show the gap between them and the normal devs.