No it isn't.
The last thing a dog wants is to fight, it only does this when cornerd and threatend.
A dog can growl and bark and bristle to scare an opponent or intruder away, but it should never bite. That should never be incuraged, that should never happen.
If it does, the owner is responsible for not training the dog or keeping some kind of muzzle on the dog.

There isn't just thiefs and murderers sneaking around, kids do that alot too. And they tend to die rather easaly from dog attacks.