The government is the biggest "en negro" employer of the country. It's also the biggest hirer of workers under precarious conditions such as fixed time contracts. Ironically, one of the biggest "en negro"-hiring branches of the federal government is the Labour Ministry. I suppose you're also not surprised by this, since you must already know the government to be the biggest everything in Argentina :P
Hiring "en negro" or under precarious conditions is not only terrible because of tax evading, in this case they would only be cheating themselves. It mainly sucks to be employed that way because you loose (or its harder for you to enforce on your employer) most of the labor benefits you usually get when you are employed by someone here. In Argentina that means no social security, health plan, granted days of leave to study for exams, extra money on holidays, bonus 13th month salary, increasing days for vacations when employed for more than 5/10 years, etc... the list is in fact pretty extensive.