There's an old saying that if you owe the bank £100, you have a problem; but if you owe the bank £1m, then the bank has a problem.
This is the reason most commonly credited as being behind the AOL/Google deal. The $300m earned from AdSense on the AOL portal was just too large a percentage to comfortably put at risk.
But there's another saying the money makes money. And it seems from the SEC filings that the deal creates the conditions for a floatation of the AOL business in the foreseeable future and if that were to happen - and if Google's other properties like Video Search really work some magic with the underused AOL content - Google's stake could end up as a real investment.
How's that for having your cake and eating it?