I think that I like the margin of error. Planning is important, but it's the unknown that makes me think a 4x4 is superior. I've seen "maintained" roads that had me wondering if my front wheel drive was going to be able to get over the humps... <img src="/images/graemlins/smile.gif" alt="" />

Anyway, I can think of many situations where superiority might play a part:
1) Traffic (go around on the little used drainage ditch or median!)
2) washout/flood - might get you through it with increased lift, etc.
3) snowdrifts, deep snow
4) mud/rock slide/earthquake damage
5) driving over obstacles that mean either safety on the other side or burning your fuel to find an alternate route (if you can even get turned around)

I'm sure there are plenty more and it's the ones I can't think of that worry me. I'm just saying that in an emergency, it's assuming a lot that any given road will be accessible and that redundant routes aren't all blocked.