Battery life is where we're at with the technology. Whether it's a gaming laptop, handheld, or tablet, the reality is they don't last on full load for more than a few hours.
Nintendo could have made the device bigger to fit more battery, but it would be called even uglier than it is already, or made it even less powerful but look at what people are saying given the current spec.
Plus as far as I know we've not got any context for battery life. There are lot's of variables, Wi-Fi enabled, screen brightness, and of course the game you're playing. Nintendo have given a guideline for a game like Zelda but again ZERO context.