Indeed, the wording of the proposition could be clearer. The notion of banning immigration law is not feasible. There are procedures and regulations in place to govern immigration and these should be followed. At some level, if we abolish all immigration rules and policies, it is difficult to envision order and transparency being evident. Having said this, I think that there should be an equal enforcement of the immigration law principles. The term of "illegal immigration" has been manipulated by some to be code for "selective enforcement" of immigration rules or allow for a form of immigration profiling to emerge. These should be eliminated in the clear and equitable enforcement of immigration law and policies.
Current immigration law, which basically says if you are not living or working here legally you must be deported, reveals a split personality in America. You often hear of people who want the immigration laws strictly enforced, and many in government saber rattle on this topic too.
You tend to hear more Republican politicians and voters who argue for more enforcement, but the Republican Party also has a very pro-business segment in its membership, and some of those industres rely on illegal immigrant labor, such as the agricultural and construction industries. Since agricultural regions tend to vote Republican also, there is a disincentive to crackdown. My point being one reason not to enforce the law is that in reality, most people don't want it enforced. Or if they do, they also don't want the price of vegetables to rise, and are generally unwilling to do the fieldwork themselves. So we can either enforce the immigration law or we can have lower prices on the goods and services we buy, but we can't have both.
It is not quite clear to me what you are proposing -- that we should have no immigration laws at all? This does not seem practical -- no country has completely free immigration and we especially can't have that now with the possibility of terrorism.
But if you are talking about reasons why the US should allow in anyone who is not a criminal or a high risk of being a terrorist, here are some arguments.
First, immigrants tend to help our economy in the long run because they are the "cream of the crop" from their countries, more or less. They are the people who are motivated enough to take risks to get a better life. This, to me, is the main reason.
Second, if we did not try to keep "regular" illegals out, we could focus more on keeping out people who are actually dangers to our society.
Third, if there were no illegals, we would probably be better off because all the immigrants would be legal and would be paying taxes.
Mind you, these are only arguments, I'm not saying I believe all of them.