I'm with you on that one. I wouldn't submit a site to Show HN without at least memcached sitting in front of it. And to run a legitimate service sans cache? That's just asking for trouble.
I don't know about that when you can simply go horizontal (automatically, even) if the need arises.
Building with cache: guaranteed added upfront cost, complexity, and time to ship, without knowing if/where caching will serve you best.
Building without cache: Guaranteed upfront savings, faster time to ship, with automatic horizontal scaling as a backstop if the need ever arises, while you work to integrate the cache.
And, remember, databases cache too. Throwing extra RAM and tweaking the DB cache config can also be a viable backstop short-term.
