but when i open some apps such as internet , ect , they stay open and i've find the battery to drain slightly quicker.Surely in theory that should make it much worse for everything.
As the apps would just keep restarting, it is android, not windows.
Plus you really shouldn't be using a task killer, especially with the GS 2 that has 1GB of RAM.
http://geekfor.me/faq/you-shouldnt-be-using-a-task-killer-with-android/