I ride motorcycles now, and I still get a lot of the upsides- open to the environment, a physical activity (though more mild than biking at a hard pace), and my stress levels are lower.
But it has one more huge upside; back when I bicycled all the time, I'd always arrive everywhere hot and sweaty. Workplaces can install showers, but what about the supermarket? The library? Classrooms? Your girlfriend's house? (which of course has showers, but back then her parents would NOT have approved)
If you're going to build a city with all of those things a few blocks away, then it's an easy ride and no big deal, but that's a pretty major overhaul. I don't see it working in existing cities. Plus, besides problems with showers, the time commitment in existing cities is pretty big. I live rather close to my job, and it would still take me 1-2 hours to get there by bicycle.
Once again, not that I love cars everywhere all the time, but it's a pretty darn big feat to switch to bicycles in many American cities.
disclaimer: I do love my bikes, I'm kind of a petrol head.