Seth Masket ponders it:
This year … neither side really has any idea what the funding situation looks like in advance. Yes, Obama and (presumably) Romney will be able to look at each other's campaign finance disclosures, but those will only reveal a modest percentage of the spending that will occur this year. Where will these Super PACs deploy their spending? What sort of messages will they convey, and will those messages be consistent with what the campaign is trying to say? How do you form a strategy if you have no idea what your opponent's capabilities are, or if you don't even know what your own capabilities are?
Elliott points out that the worst is yet to come:
[A]t least the super PACs have to reveal their donors. There’s an entire separate category of political group – so-called social welfare organizations – that are expected to blanket the country with advertising but don’t have to reveal their donors. Ever.
(Chart of top political donors, at this point in the campaign, from Mother Jones)