On off, install uninstall... same story. Same sides. hehe.
Assuming you use it wisely it helps only a tad. Assuming you are aware of what you kill off, it helps. Assuming your not running it 247 it improves performance.
If you kill off things like calendar, or clock, then you'll run into above mentioned issues alarms not firing, calendars being off. To avoid such issues, use whatever task killer program's Ingore function. Problem solved.
Performance(battery), disable their(the killer) notification status that is often a part of their use, and do not use the widget functions. That allows them to run in the background. Just use the short cut to turn the killer on. You will not see any performance degradation if you do this.
Battery life, it does help, because you kill off all things you know you dont want on. And you discover random things turning on like LastFm (I wish they would just give you the option to tell it not to turn on randomly in the background). So, yes, they can help you with battery life.
If your a nerd, or OCD, then use the app (I do). If you could care less about managing your phone, and just want to use it like a regular old common user, don't use it you'll be just fine.
Choice is yours, use it or not... to be or not to be.... that is always the question of the month