For the handful of countries that ratified the convention, migrant workers are protected from mistreatment even if they are illegal immigrants to countries. Migrant workers are three percent of the world's population and should be protected in some fashion from abuse if they are mishandled in foreign countries. Sometimes, workers don't know the laws and fall into traps where they keep being deported or simply imprisoned abroad. This convention is appropriate in that it tries to prevent harsher than normal punishments for migrant workers.
No, the UN Convention on the Rights of Migrant Workers and their Families is not appropriate in its provisions on illegal immigrants, because it does not leave each country with enough power to control their own borders. Rights of migrant workers are important, but so is the ability of a country to secure its own borders. The UN Convention on the Rights of Migrant Workers goes to far in protecting rights, so countries cannot protect themselves.