Here are several good reasons:
- "Illegal" is uselessly vague. ("You're under arrest." "What's the charge?" "You did something illegal.")
- "Illegal immigrant" is dehumanizing. Murderers, rapists, and child molesters are all legal persons who have committed illegal acts; but an otherwise law-abiding resident who doesn't have immigration paperwork is defined as an illegal person. This disparity should offend everyone on its own merits, but there's also a legal, constitutional problem with defining someone as an illegal person.
- It's contrary to the Fourteenth Amendment, which affirms that neither the federal government nor state governments may "deny to any person within its jurisdiction the equal protection of the laws." An undocumented immigrant has violated immigration requirements, but is still a legal person under the law, as is anyone under the jurisdiction of the law. The equal protection clause was written to prevent state governments from defining any human being as anything less than a legal person.
Other terms I generally try to avoid using in place of "undocumented immigrants":
- "Illegal aliens." A more pejorative form of "illegal immigrant." The word "alien" can be used to refer to a non-naturalized immigrant, but it also arrives with the context of its dictionary definition: "unfamiliar and disturbing or distasteful."
- "Undocumented workers." I use this term often to refer specifically to undocumented workers, especially in a labor context, but it is not a synonym for "undocumented immigrants." When it is used as such, it is often from people who belong to a school of thought that says that undocumented immigrants should be accepted into this country because they are hardworking. The vast majority are (they have no choice; people who cross borders to make less than minimum wage tend to be), but there are undocumented immigrants who do not fall into this category, such as children, the elderly, and the severely disabled, and they, too, need advocates.
- "Migrant workers." A migrant worker is simply someone who regularly travels in search of short-term or seasonal work. Many migrant workers are documented (quite a few are natural-born citizens), and many undocumented immigrants are not migrant workers. The migrant workers' movement certainly overlaps with the immigrants' rights movement, but it is not the same movement.