Illegal aliens do jobs Americans don't want to do, so some people want to perpetuate that relationship. Some have even used this as an argument that we shouldn't bother to enforce our own laws. Others have simply said that we should change the laws. Sam asks how that isn't just legitimating a subservient class. See also Dory's similar argument.
Now I do think a guest worker program (which would be changing the laws rather than simply not enforcing them) can accomodate this concern by ensuring that these now-legal workers would have ensured working conditions the same way everyone else does. But what it would also need to involve, out of fairness to immigrants who followed the law, is that we wouldn't be providing better conditions for those who broke the law to enter this country than we do to those who did it legally, including the drawn-out process of citizenship. But I do think a guest worker program can meet this concern. The question is really which of the proposals do that. I haven't had the time to look at any of the proposals, so I really have no idea. But those who don't advocate changing the laws but simply don't want to enforce them really do seem to me to be failing the people they're trying to help, because it will just perpetuate the terrible working conditions everyone on the left is complaining about.