- "when all they wanted to do was find a job"
This is a mischaracterization of the situation. People who are entering the country illegally are not merely looking for a job (if they're even doing that), they--at least--also want to enter a country illegally. Considering that illegal entry is the central motivation for the wall, it's a big thing to overlook.
- "Therefore, they shouldn't be punished."
A wall only punishes people who have commited the crime of trying to cross it. It's also an error to say that undermining a people's sovereignty over their land is not a "harm." It most certainly is. Just as much as theft or fraud.
- "Will the wall even stop a significant number of people?"
This is a perfectly reasonable question to ask, but it's not a point one way or the other.
- "those fences may be used against us, keeping us in"
This is a reasonable concern, but a little far-fetched on its face. I don't have a sense that a significant number of Americans are dying to go to Mexico or that there's any attempt to prevent them.
"The wall seems to be a token, populist effort unrelated to anything except an appeal to intensely emotional xenophobia."
I don't know why it seems that way to you. Nothing in your post supports this assessment.
It doesn't bother me at all if someone wants to do something illegally. The legality of an action is almost entirely distinct from its morality. Can you tell me why you think legality is so important?
Yes, how so? Who is the victim? How have they been harmed? What is the nature of the harm - physical, emotional, spiritual, mental? At which moment was it caused? Approximately how much would it cost to compensate for the harm, and what would you base that number on?
Yes it is, because if we don't know what good it will do, then our null hypothesis should be that it won't do any good. It's like if I say "I'm going to jump on the spot 10 times every morning to prevent global warming." You ask "Will that actually help?" and I say "I don't know." Is that a point one way or another?
You don't have a sense of it. I do. I've already seen many of my US friends decide to expatriate to Mexico, or other countries, because they see the growing problems of a huge, overbearing, violent government. I'm assuming from the way you're talking, you live within the borders of the empire, and you grew up there. Do you think that your opinion, based on the education you were given, and the media you're subjected to, would be an informed, objective opinion on what people with good foresight and good information want? If there were scores, hundreds or thousands or even millions of people who wanted to escape the US, how would you know? And how many people from the US do you think live in Mexico? I'll give you a hint: it's more than 2.
It seems that way because almost every time I ask these questions of people who support the wall, I get the response that people don't even know if it will work, and can't quantify why they think it's a good idea, and they can barely supply any evidence indicating why it would be a good idea.
"It doesn't bother me at all if someone wants to do something illegally. The legality of an action is almost entirely distinct from its morality. Can you tell me why you think legality is so important?"
We implicitly agree to abide by laws. Those who do not are betraying the trust of those who do. That matters to me, to the judiciary, and to every nation and people on earth. It is a harm in itself. Granted, sometimes a justifiable harm, but a harm nonetheless.
As to the harm caused by not respecting the territorial boundaries of neighbours, there is a large body of philosophical work on the subject. I invite you to read it. Key words include: property, privacy, security, and contract.
"Yes it is, because if we don't know what good it will do, then our null hypothesis should be that it won't do any good"
There's a good reason to believe it will do some good. Walls work. Look at the wall between Gaza and Egypt. It's very effective. Again, you're in this habit of wishfully shifting the argument in your favour by misrepresenting the situation. Walls have been hindering the movement of people for thousands of years. This isn't a binary where we either know precisely how effective it will be or we don't know whether it will be effective at all.
"You don't have a sense of it. I do. I've already seen many of my US friends decide to expatriate to Mexico, or other countries, because they see the growing problems of a huge, overbearing, violent government."
My anecdotal evidence \suggests precisely the opposite. The difference between me and you is that I know my anecdotes are just that.
"It seems that way because almost every time I ask these questions of people who support the wall, I get the response that people don't even know if it will work, and can't quantify why they think it's a good idea, and they can barely supply any evidence indicating why it would be a good idea."
Given our interactions so far, I don't trust your ability to judge those interactions, but more importantly, it's more anecdote anyway.