Yes, direct donors might have more direct leverage over candidates, but at least it would be transparent. And parties are an essential mechanism of American politics, as indeed of politics in all democracies. At their best—and this is important—they are not the "factions" that James Madison disliked but rather organizers of priorities, think tanks for refining policy, and they stand or fail by their conduct. Private donors in essence have no responsibility for the results of their "investments."
The sad fact is that after nearly four decades, we have not purged our politics of big donations nor cured public concerns about the excessive influence of the wealthy. Nor have we enhanced the trust factor of our elected leaders. In fact, it has gone the other way. Cynicism has deepened about our political system. But what we can say is that additional funding has changed races that would otherwise not have been competitive. And what is the alternative? Direct government funding of political campaigns, when incumbent legislators would be writing the campaign finance rules? Or requiring television companies to offer certain free time to the parties, as is done in Britain with party political broadcasts that are not very popular?
Our form of political financing probably goes under the heading of the evil of two lessers, but nobody has yet come up with a better solution.