I see that Glenn Greenwald has documented some of the outside group spending that David Brooks apparently overlooked in his NYT column. That's all well and good, but I think that this is a case where the standard treatment of the subject, including that in political science texts, obscures more than it enlightens.
The problem is that people categorize campaign money based on the legal entities under which it is raised or spent. So we talk about different organizations that can spend money: the candidates, party committees, and outside groups; and then we talk about candidates raising money from individuals, PACs, parties, and from their own self-financing. All of which is true if the questions we want to answer are about legal categories. However, more often we have questions about politics that using the legal categories prevent us from answering.
Sometimes, that's easy to see. A lot of "independent" groups are obviously party groups that have simply organized a particular way in order to exploit the way the campaign finance laws were written and interpreted. For example, Michael Crowley cites the Republican Governors' Association's plans to spend money as evidence that Brooks "lowballed" the proportion of campaign spending by outside groups, but c'mon: it's the Republican Governors' Association. In terms of the political importance of that money, I don't care whether it counts as party spending or "independent" spending -- it's obviously party money. (Actually, I'm not sure whether in fact the RGA should count by legal standards as party spending or as an independent outside group; I'd look it up, but I don't care, since in practical terms it's clearly party money).
Now, take a slightly tougher case: the money that Karl Rove associated groups are spending. Those are clearly "independent" by law. But...he's Karl Rove! Everyone knows that he's a Republican operative. In order to avoid trouble with the FEC (sorry -- should have told you to put down your drink before reading that silly clause....heh, trouble with the FEC) he may have to work "independently," but again: he's Karl Rove! Of course he can figure out how to spend his money in ways helpful to Republicans without coordinating with formal party organizations.
In fact, what we often want to know about money in politics is whether it is from parties (including the entire party network, not just formal party organizations); unaffiliated interest groups; individuals who are independent of either of those types of organizations; or the candidates themselves. Some of that is fairly easy to do (see RGA or Karl Rove, above). Some is very difficult. My buddy Casey Dominguez has done some work at sorting PACs and individuals by whether they behave as part of a party network over time, and she's found that much of the money in the system should properly be considered party money (she and I are currently beginning work on another paper on the same topic). There's also OpenSecrets, which uses the information in campaign finance filings to properly attribute individual contributors to the correct interest group.
There are times when it really is important to know how much money is from outside groups. For example, it's certainly interesting to know about money from undisclosed sources. Even here, however, I'm not sure how important it is. Disclosure is most helpful as a signal to voters about which candidates are supported by which interests. When groups with bland, feel-good names but secret donors get involved in elections, then voters can't receive that signal (note that the way this actually works is opposing candidates and neutral reporters dig through disclosure info and find things to publicize). And yet...what more would voters really learn if they knew the specific origin of money being spent by the Chamber of Commerce?
The bottom line is that we do want to know who is giving to and spending on behalf of candidates, and we should have a campaign finance system that lets us know that.
What I'd really like to see is simplification...a lot of why these outside groups exist is because the current laws restrict how party, interest group, and unaffiliated money can be raised and spent. On the other hand, I'd also like to see partial public financing, so that all major-party candidates can run at least minimal campaigns. Floors, not ceilings, plus full and fully enforce enforced disclosure -- that's a campaign finance system I could strong support.