In my early days, I collected and painted one side. But I was only into WW2 (British) and Ancients. Then a close friend moved interstate and gave me his German WW2 forces, and I acquired some more Ancient armies that happened to also be enemies. As I played rarely and against friends that did not have collections, it was handy to have opposing forces. Since getting back into gaming in the last 10 years solo I have ensured anything I get into I have opposing forces.