The Droid's sensor will adjust the screens brightness based on ambient lighting. Could just be your hand that 'blocks' it for a second, or the way light reflects.... not much you can do there.
Well, you CAN disable auto-brightness and set it to a level that you like, but auto-brightness works so well, that it actually DOES save battery. I turned it off, thinking that I didn't have a need for it to every get really bright, and set the slider almost all the way to the left...think I even just put it all the way to the left for a while.
When checking battery use percentage of "Display" having brightness not auto and down, Display used a considerable amount more battery than when I turned auto-brightness back on for a while. Can't remember the exact numbers, but I know it was a difference of at least 10-20%, enough for me to decide that auto is the way to go.
In the end, it's all about your own preference. If you'd like the brightness to stay the same all the time, disable it.
I know that, if your screen has a lot of white on it, sometimes it seems like the sensor reads that and cranks the screen brightness up even more. I've noticed that while using DroidLight [the Moto app to use the flash as a flashlight] as well.
This actually is not an accurate statement. While the auto light sensor does a good job at keeping the light at what you need it to be to look at the screen it unfortunately does not save battery compared to keeping it down. If you were using the screen for the "exact" same amount of time, one using a 25-30% setting and one using auto, the auto will always be higher.
Leaving it on it my experience, it will usually fluctuate between 40 and 65% brightness, nowhere near where i keep it. On average in a room with florescent lighting it stays at around 45-50%. For those who say, psh 15-20% is trivial when it comes to saving battery, it is not. My average battery will last now, anywhere from 12-20 hours. Thats moderate - heavy usage of the device.
If your using your droid with a usb charger into your computer, and you shut it off, it will not come back on. It will just charge like that. I'm not sure if that's intentional, so you can actually charge it faster, or not, but so far i'm sure that it works. If your using the wall chargers, it will only restart if you shut your droid off.
Taskkiller's "can" be useful to have, but don't leave them running in the background. If your going to have it, make sure that itself is one of the things you terminate. Keeping it on, and killing apps that don't need it, can cause weird things to happen on your phone, that's why most say just to uninstall them. I find them useful on a daily basis. I don't so much use it for memory management, but to kill apps that are stuck, or ones that will drain the battery. (Most have their own opinion about this, merely stating mine)
20% drain on a battery in 30 minutes is bad. Don't let anyone tell you it's normal. (I doubt anyone on here would) If you've used a task killer and made sure the task killer and all other programs were killed, and the only thing left running was the security program, then there are 3 possibilities. One is that the security app is the problem. Second is that using the task killer, one of the programs got stuck and is eating battery (it does happen). Third, you just have a bad battery, but if you stop using the security and everything is normal after that, i doubt this is the problem.
It can be good to have a spare battery if your on the go and don't always have access to a charger. My normal day is spent around computers and power outlets, so i don't ever have to worry about charging it. Some users on here spend all day outdoors doing stuff, so for certain people it's a nice thing to have a spare battery, just in case.
In your game situation, i haven't played many games, probably only about 4-5, and none of those games ever dragged after an hour or two of playing them.