Why is this an issue?

The overloading mechanism should be used in place of optional parameters for several reasons:

• Optional parameter values are baked into the method call site code, thus, if a default value has been changed, all referencing assemblies need to be rebuilt, otherwise the original values will be used.
• The Common Language Specification (CLS) allows compilers to ignore default parameter values, and thus require the caller to explicitly specify the values. For example, if you want to consume a method with default argument from another .NET compatible language (for instance C++/CLI), you will have to provide all arguments. When using method overloads, you could achieve similar behavior as default arguments.
• Optional parameters prevent muddying the definition of the function contract. Here is a simple example: if there are two optional parameters, when one is defined, is the second one still optional or mandatory?

Noncompliant code example

void Notify(string company, string office = "QJZ") // Noncompliant
{
}

Compliant solution

void Notify(string company)
{
  Notify(company, "QJZ");
}
void Notify(string company, string office)
{
}

Exceptions

The rule ignores non externally visible methods.