a) I'm the descendant of immigrants (half of my grandparents were born overseas and naturalized... my grandmother technically naturalized twice), and I can't justify it was fine for them to be here, but it's not fine for others to immigrate.
b) A lot of my circle of contacts are immigrants or descendants of recent immigrants. I can't justify it's fine for them to be here, but it's not fine for others to immigrate.
c) A large portion of the leaders of companies I've worked at are immigrants or descendants of recent immigrants. I can't justify it's fine for them to be here, but it's not fine for others to immigrate.
Following onto c, those people who immigrate here, and start companies that hire me have clearly benefited me.
I don't know how I really feel about open borders. I'm fine with reasonable policies limiting immigration. I think you'll find consensus that the current US policies on immigration are not reasonable, although there's definitely not consensus on in which direction the rules should change. I think the immigration categories are roughly ok, although there's clearly some gaming in E1; but I think the way numeric limitations work and how some categories have to wait 20 years for their priority date to come up could use some help.
Numeric limitations by country of origin doesn't feel equitable when countries have vastly different populations and land areas, it doesn't feel fair that Liechtenstein's whole population could immigrate through the numerically limited categories in 2 years, assuming sufficient slack in worldwide numbers, but most countries would take hundreds of years for their whole population to make it through the numerically limited categories.