The posters in here saying, "Well, they're here illegally. I don't see the problem" you are really ignorant to the situation. I'm not saying that insultingly -- I'm staying it in the most literal sense. You are missing quite a few pieces of knowledge and context, not the least of which is the inconsistency with which these laws are a applied.
My family immigrated illegally and are now naturalized citizens (this was way before 9/11). Many families actively contribute to the economy and to society, like mine did, and many are given a pathway to citizenship. The lawyers who help in this process expect you to be here illegally as a way to migrate. The US authorities often do, too. Being an illegal resident is a de facto part of the process for many to becoming a citizen.
When there is a rapid change in enforcement or policy, it can disrupt a good chunk of society and the economy with it. It's also a big shock on families often in financially precarious situations. That might not be your responsibility, but ultimately, it affects the health of the nation as a whole. Consistent application of laws is as important to a stable society as the laws themselves.
Some posters in here addressed the distinction between all illegal immigrants and the ones already ordered to leave. Fine, that brings some nuance to the discussion, although I still wouldn't call it open and shut. But the ones who say "illegal? Then they should be deported" are attempting to paint a real world issue in fairy tale black-and-white morality.