Consider your target audience

Jeff Atwood (CodingHorror) complained about the fact a caching addon for Wordpress is optional, and is not included in the basic software:

Personally, I think it's absolutely irresponsible that WP-Cache like functionality isn't already built into WordPress. I would not even consider deploying WordPress anywhere without it.

And why is that?

This is an incredibly scary result; blog.stackoverflow.com is getting, at best, a moderate trickle of incoming traffic. It's barely linked anywhere! With that kind of CPU load level, this site would fall over instantaneously if it got remotely popular, or God forbid, anywhere near the front page of a social bookmarking website.

I don't have a traffic report for stackoverflow.com, but since both Jeff's and Joel's blogs (both being among the most popular technical blogs in the world) point there, I suspect my definition of "moderate trickle" may differ from Jeff's.

And as Arik pointed out, Wordpress may target smaller users, for which this is not an issue (someone wrote the problem occurred with more than 20000 users per day) while power-users can install optional components to deal with the traffic issues.