[akkartik]

Because everybody has at some point tried to run rake on a new project and found some version incompatibility along the depth of the abyss^Wstack.

[steveklabnik]

Hasn't bundler solved that particular problem for a few years?

[akkartik]

Bundler isn't installed by default.

In practice I've found bundler to be just another layer that can have the wrong version: http://xkcd.com/927. But I have no interest in bringing up old painful episodes, I'm done with the ruby eco-system. So I'll just restrict my response to the previous paragraph.