The choice of words matters because they are used to convey specific meaning. They're often used for reasons that aren't immediately obvious. Perhaps in everyday language "illegals" was once accepted in polite company. However, it is now intentionally used by people who oppose not just the actions of undocumented immigrants but the people's identities and characteristics. It's a code word (i.e., dog- whistle politics) meant to convey a point of view to like-minded persons but retain a certain amount of deniability when challenged. Consider the example of the various words used to label African Americans over the years.
http://en.wikipedia.org/wiki/Dog-whistle_politics
While I typically disagree with them, there are some merits to the arguments of those who favor certain strategies with respect to enforcing immigration law or to restricting immigration through changes in legislation. However, advocating for those positions doesn't require language intended to attack and inflame.
If you disagree with those on the left on immigration policy, that's fine, but remember using words like "illegals" persuades no one to change their minds. Try switching to "undocumented immigrants" and see what happens to the tone of the conversation. I bet you'll find that it cools things down and allows for rational discourse.