I'll try to post a longer post elaborating on this, but I think that in the immigration debate, it is important to note that one reason why there is a procedure for legal immigration, rather than us just letting everyone come here, is to limit the amount of immigration. That is, we (meaning U.S.ers in general) want to be able to limit immigration so that we have a number of immigrants that we can assimilate and that we can integrate into our culture. Moreoever, we want the power to adjust immigration depending on our needs and depending on how the costs and benefits (to the U.S. and its current citizens) change over time.
Illegal immigration short-circuits that by causing our immigration levels to reflect only the desires of the immigrants themselves, and of certain businesses which benefit from the cheap labor.
The reason why this is important to point out is because a very common idea (I don't like the word "meme") that has been circulating in political circles is that the reason why we have so much illegal immigration is because we are just too darn strict in letting in legal immigrants. A lot of people talk about "securing the border" and discuss why immigration is a wonderful thing, so long as it is legal, and say that they don't have anything against immigrants, just against those who "break our laws." The subtext is that we should have a system whereby all of these people could enter legally if they so chose. In other words, immigration ought not be reduced at all, we just need to legalize those who are here.
The fact of the matter is that we need to reduce immigration so that we can assimilate those who are already here. Periodic restrictions in immigration helped us to assimilate immigrants in the past, restricitons now ought to have a similar beneficial effect. Moreover, some immigrants are good for the U.S., and some are bad (I am talking about individual immigrants here, I am not using "some" as codeword for different racial groups), regardless of whether they entered and stayed legally or illegally. The ones who have a net detrimental effect to our country ought not to be let in. For these ones, it's not that they are here illegally and that is why they are a problem. The fact is, they are a problem and that is why we refused to let them in legally.
So the point is, any good immigration reform is going to result in a lot of people who would have otherwise gotten in not getting in. Among those who want to come here, there will be losers. Moreover, the goal is not just decreasing the total amount of immigration; there will be particular people whom we decide that we want to keep out, even if there are empty slots left. So not only will there bw losers, but the U.S. government should be the ones determining who the winners and losers are (that is one of government's legitimate functions). Those who say "we don't mind your being here, just come legally," are either being disingenuous or stupid.
That is all.