In what situations is static method a good practice?

Question

I have read the following discussions:

Should private helper methods be static if they can be static , and
Should all methods be static if their class has no mem

Answer 1

Util classes can be OK to be static. As in someone elses example above, with escaping strings. The problem lies when those utils classes perform functions that we would want to mock. For example, the FileUtils class in apache commons - it is often the case that I want to mock file interactions without having to play with real files. If this was an instance class, it would be easy.