If I click on something like that, I'm expecting a page reload, not a menu.

I agree, I would expect the same behavior, but I don't think that justifies to activate a menu on a mouseover. Nobody would like an operating system that triggered menus on mouseover. It's just to easy to trigger on accident. I know I'm in the minority on this subject, because most sites do use that technique. Only site I can think of that doesn't is google (the more link off the homepage).

However, is there harm in getting dynamic behavior when you're expecting a full page load?

Moreover, it would be better for consistency between the javascript-enabled and disabled versions to use a click, which could be a minor win if one sometimes uses the site with javascript and without depending on what computer they're accessing from.

I'd be hesitant to click on a link expecting a reload if I thought that it would take me away from the main page of the site, while the hover-activated nav is no commitment.