Immigrant advocates say detention is often unmerited because, in many cases, illegal status is discovered when someone is arrested for a relatively minor offense, such as driving without a license, that wouldn't generally result in jail time.
This makes an argument only if you suspend reality and ignore the fact that sneaking into this country is a very serious crime itself. Using this logic, a rapist caught due to a traffic violation should not be arrested because his arrest for rape would be a side benefit of the traffic stop. It would be unfair to arrest the rapist for rape when the rape wasn't the original reason his identity became known.