One sets up a system to buy elections when you allow unlimited political donations.
America has a huge disparity in how much money is held by the richest and poorest. For instance, the “top one percent” holds 42% of the country’s wealth. The bottom 80% own 7% of the country’s wealth.
If you allow unlimited spending, you set up a system where that top one percent literally can outspend the rest of the country. This allows them to blanket their message in advertising, and perhaps more importantly simulate “ground game” by hiring people to go door to door (which used to be done via volunteers, so it was necessary to have a message that resonated with people, but recently Walker’s 8 to 1 advantage allowed him to just pay people to go door to door and get the vote out).
If the richest people can dominate the public sphere, and you get ten commercials for every one your opponent runs, you’re going to win some elections. This is because advertising works. It isn’t a sure thing, because some districts are simply too safe to be swayed by advertising.
But many aren’t too safe. A small electoral advantage (from unlimited money) creates a situation where the candidates chosen by the wealthy are going to be more likely to win over time. This will drive policy to favor the wealthy. Specifically, fewer regulations on their businesses and fewer taxes on them personally. Keep in mind that every tax cut will drive up the deficit or cut a program that doesn’t favor the richest people in America, further damaging the country.
Also remember that when you didn’t have environmental regulations you could set Lake Erie on fire.
Also, as it happens, this will be a Republican advantage, so you can say hello to other Republican ideas that they will now have the power to legislate. Gay oppression laws, reduced access to birth control, banned abortion, privatization of whatever programs they can get away with. And a whole lot more.
So, allowing the very wealthy to create an un-level playing field (moreso than before anyway) is a bad thing. It is against American values, and would cause significant damage to our democracy. So, it rises to the level where limiting spending (if you think spending is speech) is worthwhile, because of the damage not limiting it would do to the public good.
Remember, sedition is illegal. Why? Because it threatens the continuing government of the US. Unlimited political spending does the same.
And now Lance will scream something along the lines of, “First Amendment!!1111 Your’e an arrogant communist!!” 
They have meaning for anyone that’s honestly listening to what I’m saying.
You’ve never corrected me that I’m aware of. You keep pretending that it’s impossible to limit speech (which assumes that political donations are speech) even when I show you multiple examples of the US limiting speech.