[jriddycuz]

Isn't it bad design because it hinders testability, maintainability, and correctness?

In general, whether we are talking about procedural, OO, functional, or whatever kind of programming, code is easier to reason about and test and use if the effects of its individual units (functions, classes, modules, &c.) depend largely on the inputs to them. In functional languages this is called "referential transparency," but its value applies to OOP as well.

Using things like singletons creates hidden dependencies on things that can change over the lifetime of the program without those changes being made obvious to clients of that unit of code. Good design nails things down to explicit parameters.