OK the 20 billion dollar question, why doesn't a PA-28 have the option to select both? Was it just cost or does anyone know if there is a good reason for it?
I think it has something to do with the low wing. Because the fuel tanks are below the engine, fuel is "sucked" from the tanks by the engine driven pump (or electric pump). The problem of running on both tanks is that if one were to run dry, the pump would no longer be able to "suck" from the remaining tank.
It's hard to explain but try this...
Get 2 cups, each with a straw. Fill one cup with water and leave the other empty. Now put both straws in your mouth and suck. You should find that it's almost impossible to draw water from the cup that is full. Fill them both, and you have no problems!