Immobilizer Enough?

I am not familiar with all of the new ways that car thieves steal cars, and less than 25% of my customers are interested in alarms, so I do not have to get very creative when it comes to putting in theft-deterring features.  I have seen some people go to extremees on this forum, so i am sure there are some people in higher crime areas then myself.  My question is:  With a Transponder based anti-theft device, is a starter kill relay, or an ignition kill relay going to improve the vehicles security.  I have heard that some thieves use a huge set of keys to test in the ignition, but I am not sure how likely it is that it would work very well.  And if the thirf is good enough to get past the immobilizer, would a starter kill relay deter them in the least?  I mean they could cut the power to the alarm and they would be off.  Unless I had normally open starter kill rleays but I am not going to do that.  So what else am I not considering?  Now i know there is nothing you can do to guarantee a car will not get stolen, I mean they could tow the damn thing if they wanted.  I guess I just don't see a purpose for the starter kill or the ignition kill or the whatever kill since they are all controlled by the alarm. 

Any thoughts?

For GM it is similar to coding another key to the computer.  I don't want to say exactly how to do it but it only takes a couple of minutes.