Virtual functions in C++ make allow us to do mocking, stubbing, and faking, which helps us test code in isolation in TDD or just microtesting in general. A frequent objection to virtual functions is that it "costs more" than simple function calls.
The cost of a virtual function is hardly ever important, unless the function is being called in a very tight loop, or it is being called at interrupt-time, and even then compiler / linker optimization might overcome the "extra" table-lookup that makes a function virtual.
In C, calling a function through a function-pointer is equivalent to calling a virtual function in C++. Implementations of file-systems APIs in Linux and BSD Unix use function-pointers internally to do what C++ does with virtual functions.
The cost of cache-misses (on-chip cache and off-chip cache) is a LOT bigger than the cost of virtual function pointers. Incorrect branch-prediction is also a CPU-level cost that people fail to understand.
The best way to know if making a function virtual has any user-apparent cost, is to measure the application via some tool that does non-invasive timing. Often, there will be no visible effect if you make some functions virtual that were non-virtual before. (I'm talking about REAL applications, not benchmarks.)