I'm certainly not naive enough to think that the cards cannot be forged. I guess they make it difficult enough to deter muggins from having a go at home with his inkjet. If someone goes to the expense and trouble of either buying the equipment to print their own, or buying a forgery on line, then they are probably not the type of person you want airside anyway.
I guess it establishes a base line. If no one is wearing anything, how do you know who should be there?. A lot of places these days require people/staff on their premises to display an ID card of some sort. An ASIC is not the be all and end all either. It is just one part of the overall security system. Just because you have one, it doesn't mean you can go wherever or do whatever you want. It is only an ID card after all. It just signifies you have undergone a standardised check.