The problem with this provision is that it seems so necessary. Those evil insurance companies "wrote the bill" after all, and they must have been okay with this provision being in there.
That is a misconception of what happened. Yes, the health insurance companies saw that some national health care bill was going to be passed. Yes, they provided their input, in the form of pre-written provisions that would screw them over the least. Yes, they eventually backed Obamacare. None of them wanted it, and they would all have been much happier had it not passed.
People look at Barack Obama today, against a highly antagonistic House, and forget how much power the Democrats wielded in 2009 and 2010. Some form of Federal health care bill was going to be passed; it only made sense for the insurance companies to help craft the bill. For one thing, few people know health insurance like the health insurance companies. For another, they would really like to survive.
When combined with Guaranteed Issue, and separated from an Individual Mandate, the immediate coverage of pre-existing conditions will kill private insurance. My more-or-less educated guess is that they would not survive a decade. Today's offering at The Axiom Report (to be linked when posted there update: linked) explains what pre-existing conditions really are, how they're covered today, and why immediate coverage of all pre-existing conditions is so bad.
Click over and read the whole thing.