These guys should have taken the 4 billion from Google. Their unfair advantage is pretty much inexistent that leave them extremely vulnerable to any competition.
As for the merits of not taking the deal, I think we should be clear – the early investors and many early employees have done alright and most likely won't be losing their shirt like any of the recent investors:
http://allthingsd.com/20110602/where-did-groupons-billion-do...
Also, fwiw, this post seems to be fairly well thought out and it is clearly from before the IPO, so it's worth looking back on with fresh eyes: http://shortlogic.com/post/6142108636/groupon-ipo-pass-on-th...
This was the kind of thing that thoughtful business analysts were noticing in the weeks before the Groupon IPO: the company didn't have a sensible earnings model, even if it had "revenue" that dug it deeper and deeper into a hole.