In my opinion, this is a perfect example of what's currently very broken with the VC/funding model. Groupon is wildly successful, wildly profitable, and yet it's investor can't get paid anything without the whole company having to do this weird funding rigamarole.
In the old days they would have just gone public and everybody would be happy. But with the latest regulations nobody wants to do that anymore. And they don't want to sell their company, for good reason. So they're stuck.
What there really needs to be is some accepted, pre-defined method for investors to profit from a successful company's cash receipts. That would simplify this whole process, but I don't know how feasible that is.