Of course. But there are ways to discourage that. Again, employers are the key. If the plan says that illegal immigrants can't prove that they have worked for the same employer for a year or two, they won't be able to get a job, and will be deported, then it won't encourage many to try to get here except through the legal program.
If the program isn't made available to illegals at all, then it'll provide further discouragement to attempts to get through except through the legal program.
And how is an illegal supposed to prove that that he's been working for the same employer for all that time, if there's been no legal basis for his employment? Most illegals work under the table, do they not? They don't have SSN's, so there's no way of tracking them that way. So how is it done?
And regardless of what the legislation ends up saying, perception itself has a strong effect. Many aliens are illegally crossing into the U.S. thinking that they probably will be able to take advantage of a program like this. It has influenced their decision to come here. And the administration has made no public statements to speak of to try to convince them that they're wrong.
Above all, a program like this does not need to be passed before or at the same time as a border-security bill (again, I'm not talking in about guest-worker programs in general here, but about specifically making those programs available to illegals). First we should take all reasonable steps to stem the tide of illegal entry (something we haven't come close to doing), and then we can decide if we really need to do anything for the illegals already here. It's not a foregone conclusion that we would. Instead, we may find at that point that once the flow is cut off or drastically reduced, the sense of emergency won't be the same. We may find that we can remove the illegals currently here through a slow rate of attrition.
It's still my accusation that powerful politicians are pushing for including the guest-worker-status-for-illegals plan in a border-security bill because they know it would have no public support on its own once the border comes under better control.