I think many intruders will try to spring a frame open as it makes less
noise than smashing a window - and this country has a significantly high
level of burglaries whilst the occupants are at home.

Window contacts can detect an attempted intrusion rather than waiting
until
the intruder has entered the property, which has got to be reassuring.

ONLY IF THE WINDOW IS OPENED!!!!!! If the frame is forced from the wall - no
alarm! If the glass is broken and access made through the space - no alarm!

Also, perimeter protection means that occupants can wander around the
house
at night with the alarm (part) set - especially important with bungalows
(although a PIR in the loft can be useful in modern bungalows with tile
roofs which are vulnerable).

Yes, but the perimeter protection needs to be proper - not contacts. With
contacts, for example, you can't have a window open for ventilation while
the alarm is set. With inertias you can.

As for a PIR in the loft... yes, great idea - until summer when the loft
gets hot and the PIR goes asleep! Oh dear, you didn't know about that did
you??!!