The only reason is that I currently have 2 batteries. One on perko side one and the other on side two. I just ordered three optimas for the stereo. I hear that it is bad to mismatch batteries. I figured I would just take the two I currently have and parallel them for the boat and it's electronics.
I'd save the weight, the cable, and the battery box, and just use one of your existing batteries. No sense using something just for the sake of using something. Also, your alternator is going to take a beating trying to charge all those batteries.