it's quite simple:
people with the opportunity to advance themselves tend to be functioning, contributing members of society. if we have fewer of them, we develop something called social decline. social decline means fewer teachers, fewer qualified persons to replace retirees, brain drain in college towns and states w/ great education systems, and all around suckage.
i don't want the united states to suck because people who came here as undocumented minors aren't allowed to improve their lives. especially if they actively seek the opportunity to improve their situations -- isn't this why parents would risk legal action by the US government in the first place? to see their kids improve their situations? isn't that what any parent wants for their child(ren)? if we're allowed to go to the polls and vote away another person's rights, why can't we influence our lawmakers to grant rights to others? the right to incur obscene amounts of student debt and then work their asses off to pay it off? the right to go into the armed forces to serve in whatever capacity necessary? there are undocumented minors who want to do this. so why say no?