> But I can't think of any use case when it would be undesirable to include the extra parameters

One use case is that you are actually loosing information what arguments Signature.bind() was called with, when defaults are included. In some cases this information is important.

> hence the possibility of a separate method

Since it's relatively easy to add mix defaults in, I'd prefer to be conservative here, and wait for a stronger community interest before adding a new method to API.

But thanks for the bug report. If you find any other use cases for a separate method, please feel free to update this issue.