The iPhone 5 is now less secure because attackers now know that there is a vulnerability, and they will find it. If, previously, they had thought there wasn't a vulnerability, they might have invested fewer resources in hacking it. That is now no longer the case.

Increasing the number of people who will find exploits on a device, will reduce the security of that device.

Put another way - a platform that no (talented) engineers are attempting to find exploits for is more secure than the same platform if many talented engineers put their time into finding exploits for it.