Having a staging server doesn't only help with capacity problems, but more so with deployment problems and differences in the environment. You might be running 32 Bit on your server but 64 Bit on your dev box. Or prod uses Amazon RDS, but locally you just have MySQL on the same box. And oh suprise, now your mysql gem doesn't want to be built. Or you ran each migration on your dev box separately as you build them and when you run them all at once during your prod deploy, things blow up. That's what a good staging environment should protect you from. And for all of that to be a problem, 2 users a minute are more than bad enough. You don't need to be Facebook for that.
Even better, if you for example host your app on heroku, you get you staging and demo env for free in less than 30 minutes!
Patrick's reference to "capacity problems" was in regards to deciding whether to clear memcached. In a large scale deployment, such as Facebook, clearing the caches could overwhelm the servers. Patrick is small-scale enough to not worry about that, and prefers clearing the cache to avoid stale-cache-related bugs.
I do agree regarding how nice it is to use Heroku. My only issue with them is that they only support Ruby and Node.js, so I need to take my Clojure applications elsewhere.
