In general, a virtual fn means to start at the class of the object itself, not the type of the pointer/ref ('do the right thing based on the actual class of' is a good way to remember it). Virtual destructors (dtors) are no different: start the destruction process 'down' at the object's actual class, rather than 'up' at the ptr's class (ie: 'destroy yourself using the *correct* destruction routine').
Virtual destructors are so valuable that some people want compilers to holler at you if you forget them. In general there's only one reason *not* to make a class' dtor virtual: if that class has no virtual fns, the introduction of the first virtual fn imposes typically 4 bytes overhead in the size of each object (there's a bit of magic for how C++ 'does the right thing', and it boils down to an extra ptr per object called the 'virtual table pointer' or 'vptr').
