ISR is internationally proven to deter thieves from stealing vehicles. The idea is that all windows are permanently etched with a unique reference code. ISR database holds detailed info on the car and the registered ISR customer, and can also hold handy info like radio codes,key codes etc etc for the customer. By fitting the label, it warns thieves that all windows are marked, and that if they steal the vehicle to sell on, the prospective new buyer could ring ISR before buying, quoting the etched window markings, and they would be alerted that the vehicle is stolen etc. Many manufacturers use this system, because it does statistically reduce thefts. Thieves generally will 'move on' to the next car that doesn't have this protection.
By taking the sticker off, you are no longer warning the thief (although permanent markings are still on glass) But more importantly, you are no longer warning any prospective buyer to check the history out when they are thinking about buying your stolen pride and joy! I would keep at least one sticker somewhere obvious on a window if I were you.