<!-- --> doesn't help, because comment tags also hide content from browser :)
In principle, smart search engines should understand firm elements
like navigation bar and penalize their weight. We do that, at least.
I think you misunderstood this.
The comment tags do not hide things from the browser, think of this:
<html>
...
<!--noindex-->
stuff we want hidden
<!--/noindex-->
stuff we don't want hidden
...
</html>
... John