The "it" which I'm saying "isn't all that important" is throughput. Not biking/walking, traffic throughput.
I don't understand why everyone is coming out of the woodwork here to criticize my car-centric viewpoint and tell me how great biking or walking is. I guess it's Yet Another Example of how people on the internet can't handle the concept of disagreeing with a single point.
In the context of designing cities, traffic throughput is ABSOLUTELY important. It governs how dense the city can be and the quality of life for its inhabitants. Sure, you can have spread-out cities but that sprawl has negative effects (increased travel times, inefficient traffic patterns leading to congestion and delays, economic burden of owning a car, kids have a hard time getting around, health effects of reduced walking, etc).
Where you literally can't? No. But where current infrastructure has made it hugely impractical to? I'd say that's most of the country, especially suburbia.
"I would even argue that it isn't all that important."
If you're ok with driving everywhere, then no, you wouldn't think it was. Not everyone wants to do that, though. And making things friendly for cars tends to make it harder for walking and biking.