To STL or not to STL, that is the question...

[AvDav]

"Why are the STL classes not designed to be inherited from"
I find this inheritance useful, why not? (sure not to create its instance by new).
class Matrix : public vector<vector<double> >
{
//provide operator=()
//other operators and operator(int, int) for a[i][j]
//copy constructor
public:
double get_determinant() const;
double get_minor(int row, int col) const;
};