Here's the second reason:

Contrary to the argument made by its supporters that denying illegal aliens licenses would prevent terrorists from 'melting' into society, this legislation would guarantee a larger haystack in which terrorists can hide thus making it more difficult for law enforcement to identify them. Counter-terrorism strategy is based on reducing the suspect population so that security resources can be focused on more likely suspects. Denying identity legitimacy to 13 million illegal aliens -- the vast majority of whom are not terrorists or otherwise threats to national security -- just increases the size of the suspect pool for law enforcement to have to sort through.

Reason number three:

Third, the proposed legislation would increase the incentives for fraud by greatly inflating the value of a driver's license and by creating significant new demand for fraudulent licenses by making the driver's license actual proof of citizenship or legal status....If 13 million people living within our borders can't drive, fly, travel on a train or bus, or otherwise participate in society without a driver's license and they cannot get a legitimate one, then the market will supply them an illegal fraudulent one. State DMV bureaucracies, no matter how well- intentioned, do not have the resources, training, or skill to prevent fraud driven by this additional demand and no federal mandate will be able to prevent organized criminal elements from responding.

"On the other hand, if illegal aliens are allowed to get legitimate licenses upon thorough vetting of their identity, then the only ones who will be trying to get fraudulent documents will be terrorists or criminals -- who will face increased costs and more opportunities for mistakes if there is less overall demand - - and law enforcement resources can be focused on these activities.

Currently, 14 states allow undocumented persons to obtain drivers' licenses.

These laws were enacted for public safety reasons -- to ensure that drivers meet some standard to drive and to lower insurance premiums by decreasing the pool of unlicensed and uninsured drivers. In most cases, these laws were passed with the strong support of state law enforcement officials who recognized the advantages of being able to identify drivers and discourage unlicensed drivers from fleeing from minor traffic infractions or accidents because they were fearful of being caught without a license. The analogous arguments hold for national security -- the more we can encourage otherwise law abiding people within our borders to participate in the system the easier it will be to identify those that pose a true threat.