When using vectors for a vector drawing, the origin offset for a every vector has to be stored somewhere - why not the vector type itself?
I have a TLine type for that :-) Storing them as an angle and a magnitude gives you the benefit of being able to keep track of which direction the entity was moving in even when it has stopped moving, whereas that information is destroyed when using X and Y. It also makes it easy to slow an entity down due to friction, and adjust its direction of travel.
Good point for a tank game or something similar.