Peter,
Its a good point you make, I did know about it but suspect many don't.
I was advised a few years back that its ok to use plastic ty-raps because if they want to be in will cut them off, so thats how i deal with it.
Its more a case of just making sure it doesn't come open accidently rather than security as i carry any valuables in my hand luggage.
One point of note though is I only put the ty on the sitcase main compartment and carry a pair of nail clippers in one of the smaller pockets so i can remove the plastic ty when i arrive at my destination.
Mick