<p>I know it is a good practice to declare virtual destructors for base classes in C++, but is it always important to declare <code>virtual</code> destructors even for abstract classes that function as interfaces? Please provide some reasons and examples why.</p>

It's even more important for an interface. Any user of your class will probably hold a pointer to the interface, not a pointer to the concrete implementation. When they come to delete it, if the destructor is non-virtual, they will call the interface's destructor (or the compiler-provided default, if you didn't specify one), not the derived class's destructor. Instant memory leak.
For example
```
```
class Interface
{
   virtual void doSomething() = 0;
};

class Derived : public Interface
{
   Derived();
   ~Derived() 
   {
      // Do some important cleanup...
   }
};

void myFunc(void)
{
   Interface* p = new Derived();
   // The behaviour of the next line is undefined. It probably 
   // calls Interface::~Interface, not Derived::~Derived
   delete p; 
}
```
```