"But the alarm scared them away!"

An alarm company commercial just came on, and I had a realization beyond the standard fallacy of a loud noise stopping a violent offender.

It was the expected plot...

It's night, a woman hangs up the phone after saying something like, "Boy, I sure am alone, and completely vulnerable! I'm sure nothing will happen." She then begins some distracting task like listening to music through earphones on a treadmill, or entering a sensory deprivation chamber. BUT! Unbeknownst to her; robbers white robbers dressed all in black have been watching her, and are preparing for a daring robbery rape abduction ambiguous forced entry with unimportant intentions.

The robbers kick through the door, an alarm immediately sounds, and the robbers; obviously suffering from some kind of ear sensitivity, flee as if chased by the hounds of hell. Before the shattered splinters of the door jamb even touch the floor, the phone rings and, through some form of teleportation, is answered and against her ear.

"This is the alarm company! Are you OK?"
"Yes, someone tried to break in, but the alarm scared them away!"
"Well, you're safe now! I'm here with you in this call center several states away!"
"I feel so safe!"
"I'm going to call the police in your state, probably. See, our emergency calling system doesn't route calls as well as a local 911 call does, so we'll figure out what agency we need to call, and whether or not your street is within their jurisdiction, and since the intruders are gone now, and you are in no immediate danger; they'll be there within the next 48 hours, and write down everything that happened on a piece of paper and leave. Probably. And don't worry; since you've turned off the alarm, and your door is in splinters, the intruders probably won't return when they notice the police aren't coming, and they can enter at will without worrying about the alarm. Probably. "
"I still feel safe!"
"It's a good thing they didn't cut your easily accessible phone lines with a $2 pair of wire cutters, otherwise the alarm wouldn't have been able to notify us, and we wouldn't have been able to call you."
"I maintain my naïve feeling of safety!"
"That's the kind of piece of mind we provide ma'am. We're not heroes... We're just minimum wage call center douches in another state."

Sorry, got a little sidetracked there...

The thing I realized was that the robbers knew where she was, and knew that she had an alarm thanks to those signs that erect a magical barrier alert home invaders to factor it into their plan.

This means the robbers could have easily kicked the door open, rushed to intercept the scared woman, and forced her to answer the phone and claim a false alarm, or simply cut the external phone lines and possibly the power depending on the type of house.

Either way, this commercial is made of fail.

So, are alarms good? Yes!
They alert you to the fact that someone has clandestinely or otherwise entered your home so you can act accordingly, while alerting the police or alarm company to that fact (if placed on a backup battery, and hooked up to an unused cell phone).

Then alarms keep you safe? NO!
An alarm is incapable of causing the loss of blood pressure required to incapacitate a home invader. Therefore, the invader is free to rob, rape, murder, or abduct, until the police get around to responding to the low priority call of an unconfirmed residential alarm activation, and provide the same loss of blood pressure you should have been able to deliver in the first place.

But can't alarms scare away invaders? Maybe!
Are you willing to bet your life on it? An alarm has the possibility of scaring away intruders. A firearm (preferably a long gun), used by a trained individual (you!) has the highest probability of stopping an intruder you can get without employing explosives or a flame thrower. The best thing about using a firearm for self defense is that you are always where you are when you need help. Not across town, or even across the street. Make sure you have a firearm available, and the training to use it effectively, so you have the best opportunity to defend your life.