One other interesting conflict between the vehicular cyclists and “separationsists” is over speed. To vehicular cyclists it's important to be able to ride fast. Some of their complaints about lousy bike trails go away if you don't need to go faster than 10 MPH (I find a lot of lousy bike trails perfectly suited to running, as long as I'm going slower than 10 MPH). Many opponents criticize the importance they place on speed on separated facilities, as it doesn't really affect the majority of users.
I think this is one case where the two groups should be irrelevant to eachother. Cyclists that want to ride fast should have the right to get out in traffic in most cases; a network of well-designed pedestrianized cycle paths won't stop them. But this argument ends up getting too personal. Vehicularists tend to agitate against the very existence of any slow bike trail, while separationists discount their need for speed entirely.
I'll start with the separationists. Discounting people's desire to get places quickly is a great way to become irrelevant. Your city may be beautiful, biking may be fun, but if you're biking for utility, you're going some place, and you'd probably like to spend less time getting there. Also, riding fast is just a fun thing to do. There's a more political argument that fast travel just promotes sprawl and greater distance, so it's a zero-sum game, and some of these sorts promote deliberately slowing down cyclists. I think this misses the mark. Fast bike travel won't promote any more sprawl than we already have because cycling is not the fastest way to get most places, and not how most people travel. That's the car. So fast car travel, and occasionally fast mass transit, can promote sprawl in ways fast biking, by itself, won't.
On the other hand, vehicularists don't recognize that for lots of people a slow trail is better than no trail. The numbers don't lie on this. They may be right to choose the road for themselves (I do this regularly), or even recommend it to others and teach road skills, but wrong to claim it's the only way to ride. They're right to want to ride fast sometimes, but wrong to believe only fast roads are good roads. It's another manifestation of their tendency to ignore the benefits of having more cyclists out there.
Seattle has plenty of fast cyclists zipping around the arterial streets, and they (we) aren't going anywhere. We could use lots more cyclists on the side streets and bike boulevards, using the (sort of lousy) bike trails (with caution), and rolling slow along Broadway when the cycle track goes in. Even if I won't use the new infrastructure all the time, I support it when it's designed well for its intended users.