There is a well written post over at Smashing Magazine by Louis Lazaris titled The Case Against Vertical Navigation. I have made this argument to my own clients (and other web professionals) many times, often with feedback that implies the client knows how users actually surf. This article wraps up the same points I've made and provides some pretty screen captures. The core points follow...
It Discourages Information Architecture
Using a content management system is the best way to demonstrate how true this is. Sites are no longer made up of a few top pages and perhaps one level of sub-pages. A well-structured press section (with archives and categories) will blow up a left-side navigation structure pretty quickly. It also encourages site authors to keep adding top-level pages until the navigation bar is too long to be useful.
It Wastes Prime Screen Real Estate
So much of the valuable page real estate is taken with vertical navigation that the site will miss out on key promotions or cross-links, partly because authors get more cavalier about the length of words when they are stacked in a column. On longer pages, often the entire column is wasted with dead space below the navigation.
It Doesn't Conform to Real-Life Reading
Vertical navigation results in shorter line-lengths for content, especially if you add images, pull-quotes, cross-sells, blogrolls, and so on to the other side of the page.
Fly-Outs Aren't as Usable as Drop-Downs
Many clients opt for fly-out menus to alleviate the issue with multi-tiered sites, but users don't navigate them as well as fly-outs from a horizontal menu (what the article calls drop-downs). Not only is it physically more of a challenge with a narrower active area, but adding another level gets even more complex for users.
It's Not as Successful, According to Studies
Our own user group studies have shown this, and the article references some research and shows some heat maps that indicate where users look on a page.
The Few Benefits Are Negligible
Having very long names as the primary navigation links can seem like a benefit, but in reality users don't read the entire text, they just parse it for the nouns or action words. The ability to have as many links as you want is also a detriment as users eventually can't keep track of all the options.
Exceptions to the Rule
The author does provide some examples where this rule can be broken. There are some screen shots worth checking out that support the exceptions.