I've never had a car broken into despite living in a couple of dodgy areas in Nottingham, Leeds and London, and that includes a Mk2 GL (had nothing worth stealing though), Mk3 16v and a Mk2 16v (although someone did try, but didn't get far).
But my brother just had his Mk3 Astra broken into and the stereo stolen, they just pushed the front window down and grabbed the stereo. But there wasn't an alarm on the car, and he did leave the front on the stereo.
I think it's just a case of if they see something worth stealing, and there's no one around, they'll give it a go.
But yeah the penalties for stealing are absolute rubbish, certainly not enough of a deterant.