I can see where'd it be a good deal if you could use the battery for a couple hours. Then, when you get back home, remove it and recharge it with a charger. It'd really be neat to be able to install something that would monitor your car'* power consumption/generation and charge the battery here and there when it'* most efficient to do so.
Either way, it'd be nice to be able to crank your stuff up and not have to worry about any other electrical system in the car, cause if you people run stuff like me, I do "bragging rights/loud music" for about 5-10 minutes, and then that'* enough of that, back to normal bass. It'* not really worth the strain on the electrical system.