Jock ,
My cats, like yours, tend to come in to "use the restroom." When they do poop outside, it's usually in my yard. (One of the two actually leaves hers on top of the lawn, like dogs do [no pun intended], as territorial marking.) I'm the one who has to deal with it -- as well as the occasional doggy deposit.

In spite of the fact that I have two cats of my own who spend a good deal of time outdoors in good weather, several neighbors have indoor-outdoor cats, and we have our share of strays in the neighborhood who visit my porch, I find very few 'deposits' either in my garden or on the lawn, and I hear hardly any complaints from any neighbors about any cat problems.

GUEST ,
("Cats...cause far much more damage than rats, mice etc." Maybe. Because there are enough cats around to keep the mice and rat population in control.

As ClintonH. points out, "... they GUT the local population of alley vermin..." .
You can be that if cats didn't help control the population of pigeons and rodents, ANIMAL CONTROL would soon be out there with the poison birdseed and rat pellets.


Re cats as hunters, the several cats I've had over the years have been known to kill mice, pigeons, and one garter snake (which I stupidly tried to set free by turning loose in my yard). The pigeons always get eaten. The mice are given me as tribute. The snake was just left there (probably because it didn't taste good).



Grab,
("...if I could find the owner of the cat(s) that crap in our garden, I would happily place every piece of cat shit I find on their doormat...".
That seems a much more appropriate act of revenge than trying to booby trap the cat door with broken glass.
Since when does "an eye for an eye" call for capital punishment or maiming as the penalty for petty vandalism? (Even if you could apply that label to a defecating cat.)


Liz the Squeak ("I can't help feeling that [leaving broken glass in front of the cat door] is a little small compared with some of the greater cruelties that man inflicts on man...")
Well, yeah, in terms of magnitude, and in terms of direct relevance to our own kind. But cruelty is cruelty. A person who will deliberately inflict pain on another sentient being--be it a possum, a cat, a rabbit, or whatever--is potentially a threat to us human types, too. They may not hurt animals they like or people they're not mad at, but watch out if they define you as an enemy.

Genie