A banner may contain more than just a menu -- e.g. links to important documents, content callouts, etc. However, I've found that banner content will not appear in search results. So, if I have important content that exists only in a banner, people cannot find it through search.
I've gotten around this by adding the link to the file in the main body content, but it looks much better in the banner. Our site visitors rely a lot on search, so I had to make the trade-off.