The key difference is that the Tancredo plan would not legalize illegal aliens.
Are there jobs that Americans don't really want to do, which aliens would like to do? Yes. Therefore, let the market be what the market is.
To a point. However, there are enough workers in other countries who could replace every American for a lower wage. Would you leave the international labor market entirely unrestricted?
No. I think we can maintain a good idea of what kinds of jobs Americans want to work and what we're unwilling to.