Under voltage=lower bulb life and less light. I used to work for OSRAM Sylvania, but since I know you won't listen to me, here's the first link I found:
The Truth About Daytime Running Lights | The Truth About Cars
I don't see how the cost, weight, and loss of storage of extra batteries or the possible reliability issues/extra vigilance necessary to eliminate the alternator is worth it. To each their own, but it seems like spending a dollar to save a dime to me.