Sheesh. The point is to have the 911 center receive location info on a VOIP caller under *most* circumstances. If the caller wants to conceal their location (e.g. by blocking) they wouldn't be calling 911 for service to their location anyhow, would they? And the worst-case fallback is, to pre-911 days, when the caller has to tell them where they are calling from. The vast majority of callers, after all, DO know where they are.
Of course it's not perfect. There will be people who fall between the cracks. Of course it can be defeated. I can think of a few ways to defeat or falsify ANI info too (and if you read this group regularly, you probably can also).
I don't think the special case is particularly likely. But if you do, make the alternate 911 line an 800 number, which receives ANI. Easy, eh?
There comes a point where the money spent covering any possible eventuality greatly exceeds the value. A friend of mine who works in public health bemoans the fact that the "war on terror" spends close to a billion dollars in anthrax-detection gear, to save an estimated 4 lives per year, when the same amount of money could save thousands, even tens of thousands, of lives if spent on less sexy health measures. This is like that. In real life, there's always a "close enough".