There are many many reasons to oppose illegal immigration and I've heard almost all of them except one: what role does illegal immigration play in the ongoing corruption of the Mexican government?
By not enforcing our borders we are contributing to the continued corruption within the Mexican government. We do this by allowing Mexico to export the people that, if forced to remain home, would demand change and thus force Mexico to open its economy, root out corruption, and stop the addiction to remittances from the US that result in Mexico actually assisting illegal immigrants in their trek across our border.
Instead we act as a relief valve to the social pressures that would, and should, form in Mexico.
We do a disservice to Mexico and ourselves by not forcing them to confront their problems. If we don't force Mexico to reform how long will it be before all 100 MILLION Mexicans are in the US demanding "rights" and citizenship?