I'm (currently) a postman, I think I'm a not bad one, and I absolutely wouldn't do the leaving-card-without-knocking thing.
But a wee bit of me understands why some are tempted.
The workload that is expected of you these days isn't really do-able and people seem to be pushed, often with the managers encouragement, to find ways to cut corners.
I sometimes get finished with my last (out of 6-11) 20+ kilo bag of mail, having to also carry two extra bags of 'not in' packets back to the local post office, and thats sometimes unmanageable.
This sort of thing is only going to get worse however, since money is constantly being taken from frontline delivery (the efficient bit) to bail out processing (the lossmaking bit) and of course, to provide the subsidised service for our competitors