We get a lot of customers in the shop who bring in their Android handsets complaining of freezing and poor battery life. More often than not, they are running task killers. Usually a friend or a salesperson has advised them that this is the best way to improve battery life. Unfortunately these programs do not do what they advertise as Android was designed to manage tasks and applications on it's own without the help of these programs. In the end a loss of stability and a loss of battery life from continually restarting processes occurs in handsets running version 2.2 or higher.

This article provides an excellent in-depth explanation: http://geekfor.me/faq/you-shouldnt-be-using-a-task-killer-with-android/