(Clearly there's a simple argument against MayDay PAC itself, that it might be unsuccessful in its mission, but that's a much weaker argument.)
As far as I understand it, there are four main arguments for money in politics:
1) Money is speech; protecting free speech is important, and protecting political speech (and thus financial donations to campaigns) is probably the most important form of protected speech.
2) The utilitarian argument: if money is taken out of politics, groups with less economic power but greater organization and numbers will become dominant. (I'm not sold on pure democracy either). Control of "large audiences" without control of money to reach the public might become important; I'd rather live in a world where I can donate hundreds or thousands to EFF to influence policy vs. a world where Pat Robertson or Oprah are the only people who can influence voters.
3) The implementation argument: if this eliminates a fairly accessible form of financing (the PAC), those who control smaller but more enduring forms of money for political contribution will rise in power. IMO, those people tend to be much more objectionable than people who contribute to PACs. Or, if companies are kept out of politics, but unions are allowed to continue donating, etc.
4) Priorities: this might NOT be the most important issue right now. Maybe immigration is. Maybe not going to war in Iran is. etc.
I don't know how valid these arguments are.