Mason's question was about multiple URLs. IOW, "why does basecamp.37signals.com share a load balancer with campfire.37signals.com?"

It's usually for cost reasons. Hardware load balancers are expensive, and the job they do isn't especially computationally expensive, so it's generally accepted to host multiple VIPs on a single load balancer.

Looking at it the other way, being 100% down on one LB might be preferable to being 50% down when a single LB out of two fails. The alternative gives you twice as many chances for failure, even though each chance is half as "bad". Common-mode failure mechanisms would of course still bring you entirely down. (Bad config by a human being the most common cause.)