Picture the scene....
You fit your "high end" Accenta alarm, to your house
For what ever reason you've got dishonest dan the decorating man doing the dinning rm.
Dan knows superfresco goes on easy its by graham & brown..... & should he open your untampered pir & twist the wires togethers he can bypass the 12hr circuit & come back & smash your back door in & the alarm won't make a sound
eg - tamper circuits detect people tampering with the alarm whilst its unset ?