IF and only IF there was a strong need would I buy one of those.
Like for instance I was going on a long camping trip and not planning on being near a charger for a week or maybe two? Then it would be okay.... but the price is sure right... stuff it in the glove box and who knows, it just might save you one day.
I am getting 3 to 4 days of normal use with the stock battery, so for me, it would be over kill.