There is no problem if you un-include it from your site. As you say, the session will timeout by itself anyway.
In high-security situations though it's preferable to do an explicit logout. During the period you are logged in it is possible to use your session number, even if you have left the PC. Now where security is an issue you'd be using SSL anyway, so this window is very small. But you'll notice your bank encourages you to explicitly logout. The window is very small, but it exists, so it's safer to close the window.
That said the window is _very_ small, timewise, and geographically. People leaqve their banking web sites every day without logging out.
Cheers
Bruce