After using Brainstorm myself...

Brainstorm does seem to lend itself more to having a broad general desire and hashing out the best way of making that desire happen whereas an enhancement request sort of denotes that the user knows precisely what needs to be done and how. It feels much more targetted, whereas Brainstorm feels like "let's ask the collective what is the best way to go about this goal".

In this sense, I feel both have a place. Perhaps once a consensus is formed in Brainstorm, an enhancement bug should be filed pointing to the Brainstorming session.

Just an idea.