The reason that there is little national consensus on eliminating the labor pool that these guys provide is, I suspect, that your premise is wrong, that their labor comes at the expense of someone else.
Furthermore, the issue about mandatory provision of social services to illegal immigrants is a separate issue from that regarding the effects on labor markets.
No, the point I'm addressing is that under the Bush plan illegal aliens would have an advantage in being accepted to the "guest worker" program because they are already matched with employers by virtue of their illegal employment. Law abiding foreign nationals never had a chance to compete for those jobs, and would therefore be at a disadvantage.
So, a guest worker program that legalizes illegal aliens does so at the expense of those who haven't broken our laws.
In addition, to reward a behavior is to encourage it.