A Purdue University study has shown that free mobile apps on average drain the battery quicker than the paid equivalents. The Indiana-based university researchers found that the third-party advertising services use up to 75% of the app's power drain. Take for example Angry Birds: the game itself only used 20% of its total power draw, 45% was used to find the user's location to provide targeted ads, with the remaining 35% powering said ads.
Additionally, the study went on to find another problem called a "3G tail" which is where the 3G connection is left open for up to 10 seconds after a download is completed. This results in a needless loss of power and opens up the possibility for extra data charges for the user. I understand the need for advertising to maintain free development, but battery optimization really needs to take a priority when you see numbers like this.