If you have a 357 why would you go out and buy a 38?
Having a 357 is like having two guns, two guns in one. You have an option of what you want to shoot from it. You have an option of what you want to spend on ammo.
The point is you have the choice of shooting either or. And with the government being the way they are and ammo prices being the way they are, it is good to be able to shoot lots of 38 for the practice, some 357 for the familiarity and then carry the 357 as CCW.